docs: ADR-0006 Amendment 1 — undo/snapshot design (every-mutation, hybrid, batch) + plan

Settles the undo/snapshot half (U1/U2) before implementation:
- every-mutation single-step undo (supersedes destructive-only model)
- hybrid whole-project snapshot (db backup API + yaml/csv copy),
  reconciling ADR-0006 with ADR-0015's derived-db model
- persisted N=50 ring; redo discarded on new work
- batch ops (replay + future) record one undo step; import excluded
- --no-undo disable switch
Adds the implementation plan and updates README index, requirements
U1/U2, and CLAUDE.md in lockstep.

docs/adr/0006-undo-snapshots-and-replay-log.md

@@ -87,3 +87,123 @@ authoritative state of the project (that lives in `project.yaml`
   with no additional storage commitment.
 - Tutorial authors gain a natural "starter script" format for
   exercises.
+
+## Amendment 1 — Single-step undo: every-mutation snapshots, hybrid storage, batch granularity (2026-05-24)
+
+The replay/journal half of this ADR (U3/U4) shipped via ADR-0034. This
+amendment settles the **undo/snapshot half (U1/U2)** before
+implementation, and **supersedes the original Decision's
+"snapshots only before destructive operations" model** and its
+confirmation rationale. Written with explicit user approval; the
+implementation plan is `docs/plans/20260524-adr-0006-undo-snapshots.md`.
+**Not yet implemented** at the time of writing — this records the
+agreed design.
+
+### Snapshot scope — every mutation (single-step undo)
+
+The original Decision snapshots "before any destructive operation —
+`DROP`, `DELETE`, `TRUNCATE`, schema-rebuild migrations, restore" and
+explicitly treats inserts/updates/schema-additions as *non-destructive
+work between snapshots*. That is **replaced**: a snapshot is taken
+before **every** data/schema mutation — insert, update, delete, drop,
+all DDL, and all SQL DML. Undo therefore behaves like a familiar
+single-step "undo my last command" (Ctrl-Z), which is the right model
+for this teaching environment (clarity over micro-optimisation, per
+the project's "pedagogy wins ties" posture).
+
+A consequence is that the original confirmation clause — "counts of
+rows added/modified/deleted per table and any schema changes since the
+snapshot" — **collapses**: with a snapshot per command there is no
+intervening un-snapshotted work, so undo rolls back exactly one
+command and the confirmation simply **names that command**. No
+db-diff machinery is needed.
+
+### Confirmation rationale
+
+The original justified the always-on prompt by "undo is rare and
+consequential." Under single-step undo, undo is *more frequent*, but
+the prompt is **kept** anyway, now justified by "the prompt names the
+exact command being undone." There is still no flag to suppress it.
+`undo` and `redo` each confirm (`Y` confirms; `N`/`Esc` cancels),
+mirroring the existing `rebuild` modal. The redo prompt names the
+command that will be re-applied.
+
+### Snapshot mechanism — hybrid db + text (reconciles ADR-0015)
+
+The original specifies SQLite's online backup API. Since then,
+ADR-0015 made `playground.db` a *derived* artifact with
+`project.yaml` + `data/*.csv` as the authoritative source, committed
+last for crash recovery. A db-only restore is therefore no longer
+sufficient on its own. The agreed mechanism is a **hybrid
+whole-project snapshot**:
+
+- the database is copied via the **online backup API** (honouring this
+  ADR; it is also the only safe way to copy a live database), **and**
+- `project.yaml` + `data/*.csv` are copied as inert files.
+
+Undo **restores all three directly** — no rebuild, no re-derivation —
+re-establishing a consistent `(db, yaml, csv)` triple. This satisfies
+both this ADR (the backup API *is* used) and ADR-0015 (text remains
+authoritative). The snapshot is staged *before* the mutation's
+transaction and finalised into the ring *after* the database commit,
+preserving ADR-0015 §6's commit-db-last ordering; a rolled-back
+operation leaves no snapshot.
+
+### Storage and lifetime — persisted ring, N = 50
+
+Snapshots are **persisted on disk** under the project in a
+`.snapshots/` directory and survive quit (undo works after reopening).
+The ring keeps the most recent **N = 50** snapshots (the original's
+N = 10 is raised, since single-step undo means N counts *commands*;
+still a single tunable constant), evicting the oldest on overflow.
+`.snapshots/` is added to the `.gitignore` template, **excluded from
+`export`** (like `playground.db` and `history.log`), and on the
+temp-project cleanup allowlist so an otherwise-empty temp carrying a
+snapshots directory remains safely deletable.
+
+### Redo
+
+`redo` is supported (as the original states). New semantics are
+pinned: **the redo stack is discarded on any new mutation** (standard
+linear undo/redo). Each undo pushes the pre-undo state so redo can
+restore it.
+
+### Batch operations — one undo step; `import` excluded
+
+A single user command that runs many sub-operations — `replay` today,
+and any future in-project batch command — records **one** boundary
+snapshot for the whole batch (not one per sub-command), via a
+Begin/EndBatch worker primitive that suppresses per-command staging and
+finalises a single ring entry only if ≥1 mutation actually ran. This is
+a performance win (a long `history.log` replay is one database copy,
+not N) and the consistent reading of "one undo step per user command."
+
+`import` is **outside** the undo model entirely: per ADR-0015 §11 it
+creates a *new* project and switches to it, leaving the current project
+untouched on disk, so there is nothing to snapshot and it takes no undo
+step (the new project simply starts with an empty ring). Project-switch
+navigation undo ("go back to the previous project") is a separate,
+out-of-scope mechanism — the prior project is intact and reachable via
+`load` / `--resume`.
+
+### Disable switch
+
+A `--no-undo` CLI flag turns snapshotting off entirely (zero
+per-command overhead), as a hardware escape hatch should per-command
+snapshots prove too heavy. When set, `undo` / `redo` report that undo
+is turned off. CLI-only for v1 (no in-app toggle).
+
+### Consequences
+
+- Per-mutation snapshotting costs one database backup + a text copy per
+  command; a bulk paste of N inserts makes N snapshots. Bounded by the
+  N = 50 ring and the `--no-undo` escape hatch; the ADR-0015 "batch"
+  command remains the future remedy, and a hardlink/copy-on-write dedup
+  of unchanged files between consecutive snapshots is a possible future
+  optimisation (not v1).
+- 50 × (database + text) of persisted snapshots can reach tens of MB
+  for larger projects — an accepted, bounded cost.
+- `Cargo.toml` gains the `backup` feature on `rusqlite`.
+- The Phase-3 N/A matrix row ("auto-snapshot fires for SQL DML the same
+  as DSL") becomes non-vacuous: the snapshot hook lives in the worker
+  dispatch and covers DSL and SQL mutations uniformly.

docs/adr/README.md

@@ -11,7 +11,7 @@ This directory contains the project's ADRs, recorded per
 - [ADR-0003 — Input modes and command dispatch](0003-input-modes-and-command-dispatch.md)
 - [ADR-0004 — Project file format](0004-project-file-format.md)
 - [ADR-0005 — Column type vocabulary](0005-column-type-vocabulary.md)
-- [ADR-0006 — Undo snapshots and replay log](0006-undo-snapshots-and-replay-log.md)
+- [ADR-0006 — Undo snapshots and replay log](0006-undo-snapshots-and-replay-log.md) — **Accepted**. The **replay/journal half** (U3/U4) shipped via ADR-0034; the **undo/snapshot half** (U1/U2) is settled by **Amendment 1 (2026-05-24)** but **not yet implemented** (plan: `docs/plans/20260524-adr-0006-undo-snapshots.md`). Amendment 1 **supersedes the original "snapshots only before destructive operations" model**: a snapshot is taken before **every** data/schema mutation (DSL + SQL) for familiar single-step (Ctrl-Z) undo — so the confirmation collapses to *naming the one command being undone* (no db-diff). Snapshot is a **hybrid whole-project copy** — database via the online backup API **plus** `project.yaml`/`data/*.csv` as files — reconciling this ADR with ADR-0015's "text is authoritative, db is derived"; undo restores all three directly. Staged before the mutation's transaction, finalised after the db commit (preserves ADR-0015 §6 commit-db-last); rolled-back ops leave no snapshot. **Persisted** ring under `.snapshots/`, **N = 50** (raised from 10), git-ignored + export-excluded + temp-cleanup-aware. `redo` supported, **redo stack discarded on new work**. **Batch ops record one undo step** (`replay` + future batch via a Begin/EndBatch worker primitive); **`import` is outside undo** (it switches projects per ADR-0015 §11, leaving the current project untouched). A **`--no-undo` CLI flag** disables snapshotting (hardware escape hatch). Adds the `backup` feature to `rusqlite`
 - [ADR-0007 — Sharing and export](0007-sharing-and-export.md)
 - [ADR-0008 — Testing approach](0008-testing-approach.md)
 - [ADR-0009 — DSL command syntax conventions](0009-dsl-command-syntax-conventions.md)

docs/plans/20260524-adr-0006-undo-snapshots.md

@@ -0,0 +1,482 @@
+# Plan: ADR-0006 undo / snapshot half (U1/U2)
+
+**Date:** 2026-05-24 · **Author:** session 36 · **ADR:** 0006
+(undo snapshots and replay log) — replay half (U3/U4) shipped in
+ADR-0034; this plan covers the **undo / snapshot half (U1/U2)**.
+
+**Status of this doc:** draft for review. The load-bearing design
+calls below were confirmed with the user before writing (see §2);
+the second-order calls in §10 are recommendations awaiting sign-off.
+No code is written until this plan is approved.
+
+---
+
+## 1. Baseline
+
+- `cargo test`: green (exit 0). Handoff 35 records **1659 passing /
+  0 failing / 0 skipped / 1 ignored** (the `friendly/mod.rs`
+  doctest). This is the Phase-1 baseline Phase-5 is measured against.
+- `cargo clippy --all-targets -- -D warnings`: clean (per handoff).
+
+---
+
+## 2. Decisions locked with the user (do not re-litigate)
+
+All confirmed this session via direct Q&A.
+
+1. **Snapshot mechanism — hybrid whole-project snapshot.** A
+   snapshot stores the database (copied via SQLite's **online backup
+   API**, the only safe way to copy a live db) **plus** the
+   authoritative text (`project.yaml` + `data/*.csv`, copied as inert
+   files). Undo **restores all three directly** — no rebuild, no
+   re-derivation. This satisfies **both** ADR-0006 (backup API *is*
+   used) **and** ADR-0015 (text remains the authoritative copy), so
+   the *mechanism* needs no ADR amendment. Rationale for storing the
+   db too (rather than text-only + rebuild): the user prioritised a
+   direct, robust restore for a safety feature over the smaller disk
+   footprint of a rebuild-on-undo approach.
+2. **Lifetime — persisted on disk, ring capped at N = 50.** Snapshots
+   live under the project directory and survive quit (undo works
+   after reopening). The ring keeps the most recent **50** snapshots;
+   the oldest is evicted on overflow. `50` is a single tunable
+   constant.
+3. **Redo — discarded on new work.** Standard linear semantics: any
+   new mutation after an undo clears the redo stack.
+4. **Scope — every mutation (single-step, Ctrl-Z-style undo).** A
+   snapshot is taken before **every** data/schema mutation (insert,
+   update, delete, drop, all DDL, all SQL DML), not only before
+   destructive ops. The user judged this the right model for the
+   playground's teaching philosophy. **This supersedes two pillars of
+   ADR-0006** and requires an amendment (see §4).
+5. **`undo` requires confirmation** that names the command being
+   undone. (Confirmation is kept per the user's explicit instruction,
+   even though single-step undo is more frequent than the ADR's
+   original "rare and consequential" framing.)
+6. **Disable switch — in the picture from day one.** A way to turn
+   undo off entirely, so per-command snapshotting can be avoided if it
+   proves too heavy on small hardware. Shape: a `--no-undo` CLI flag
+   (CLI-only for v1, no in-app toggle — confirmed).
+7. **Batch operations record ONE undo step.** A single user command
+   that executes many sub-operations — `replay` today, and any future
+   in-project batch command — takes **one boundary snapshot** for the
+   whole batch, not one per sub-command. Undo rolls the batch back
+   wholesale. This is both a perf win (replaying a long `history.log`
+   is one db backup, not N) and the consistent reading of "one undo
+   step per user command." **`import` is *not* a batch op** — it
+   creates a new project and switches to it (ADR-0015 §11), leaving the
+   current project untouched on disk, so it sits **outside** the
+   per-project undo ring entirely (the new project just starts with an
+   empty ring). Project-switch-level "undo back to the previous
+   project" is a separate, out-of-scope mechanism.
+
+---
+
+## 3. Phase 1 — Requirements checklist
+
+From ADR-0006 (as amended), the user decisions in §2, and the
+project's working standards.
+
+### Functional
+
+- [ ] **R1** Auto-snapshot before every mutation (DSL + SQL), gated by
+  the enable flag.
+- [ ] **R1b** A **batch** command (`replay`, future batch ops) records
+  exactly one boundary snapshot for the whole batch (not one per
+  sub-command), via the Begin/EndBatch primitive; finalized only if
+  ≥1 mutation actually ran. `import` takes **no** undo step (project
+  switch, not a current-db mutation).
+- [ ] **R2** A snapshot captures db (backup API) + `project.yaml` +
+  `data/*.csv`, atomically, representing the **pre-mutation committed**
+  state.
+- [ ] **R3** Snapshots are committed into the ring **only after** the
+  triggering mutation commits successfully; a rolled-back op leaves no
+  snapshot. Preserves ADR-0015 §6 commit-db-last ordering.
+- [ ] **R4** Persisted ring buffer under the project dir, cap 50,
+  oldest evicted on overflow.
+- [ ] **R5** `undo` app command (both modes, no sigil): confirmation
+  modal naming the command to be undone + its relative timestamp →
+  on confirm, restore the top snapshot (db + text) → pop the ring.
+- [ ] **R6** Undo pushes the pre-undo (current) state onto a redo
+  stack so `redo` is possible (ADR-0006). Redo restores it.
+- [ ] **R7** `redo` app command restores the top redo entry. New
+  mutation clears the redo stack (R-decision 3).
+- [ ] **R8** Undo/redo into an empty stack reports a friendly note
+  ("nothing to undo" / "nothing to redo"); never errors.
+- [ ] **R9** Disable switch (`--no-undo`): when set, no snapshots are
+  taken (zero per-command overhead) and `undo`/`redo` report that undo
+  is disabled.
+- [ ] **R10** Restore correctly re-establishes a consistent (db, yaml,
+  csv) triple, including the NULL-vs-empty CSV distinction and
+  internal metadata tables (`__rdbms_playground_*`).
+- [ ] **R11** After undo/redo, the TUI refreshes its view (re-query)
+  so the user sees the restored state.
+
+### Cross-cutting / integration
+
+- [ ] **R12** `undo`/`redo` added to `runtime::is_app_lifecycle_entry_word`
+  so **replay skips them** (ADR-0034 Amendment 1) — and to the
+  completion entry-keyword set (`src/completion.rs`), which the
+  function's doc-comment says must stay in lockstep.
+- [ ] **R13** Snapshots dir added to the `.gitignore` template
+  (`src/project/mod.rs`), the **export exclusion** (`src/archive.rs` —
+  like `playground.db` and `history.log`), and the **temp-project
+  cleanup allowlist** (`safely_delete_temp_project`) so an otherwise-
+  empty temp carrying a snapshots dir is still safely deletable.
+- [ ] **R14** `Cargo.toml`: add `"backup"` to the `rusqlite` feature
+  list.
+- [ ] **R15** Engine-neutral user-facing strings (ADR-0002): no
+  "SQLite" / "backup API" / "PRAGMA" in notes or modal text. Use
+  "snapshot" / "the database".
+
+### Documentation
+
+- [ ] **R16** Write an **ADR-0006 amendment** (see §4); update
+  `docs/adr/README.md` in the same edit (ADR-0000 index rule).
+- [ ] **R17** Update `docs/requirements.md` U1/U2 from designed →
+  implemented; update `CLAUDE.md`'s ADR-0006 "Designed; not yet
+  implemented" line.
+
+### Testing (ADR-0008 four tiers)
+
+- [ ] **R18** Tier 1 (unit/pure): ring eviction, redo-clear-on-new-
+  work, mutation-vs-readonly classification, modal key handling,
+  command parsing, disable-flag gating, CLI parse.
+- [ ] **R19** Tier 2 (insta snapshots): undo/redo confirmation modal
+  rendering; disabled-undo note.
+- [ ] **R20** Tier 3 (integration, real db + temp project): snapshot
+  taken on mutation; undo restores db **and** text; redo; ring
+  eviction at the cap; redo cleared by new work; `--no-undo` takes no
+  snapshots; persistence-ordering invariant preserved.
+- [ ] **R21** Full-stack flow (per global standards): bootstrap a real
+  project, run a sequence of DSL **and SQL** mutations, undo/redo
+  across both, verify read-model + on-disk yaml/csv/db all consistent,
+  reopen the project and confirm the ring persisted.
+- [ ] **R21b** Batch: a `replay` of a multi-command file produces
+  exactly **one** ring entry; undo rolls the whole replay back; an
+  all-skips replay leaves no undo step; `import` produces no undo step
+  in the current project.
+- [ ] **R22** Phase-3 N/A matrix row closed ("auto-snapshot fires for
+  SQL DML the same as DSL") — now non-vacuous.
+- [ ] **R23** End state: all green, **zero skips**, no regression vs
+  the §1 baseline.
+
+---
+
+## 4. ADR-0006 amendment scope
+
+The every-mutation decision (§2.4) contradicts ADR-0006 as written,
+so an amendment is required (the ADR is *Accepted* — we amend in place
+with a dated note, per the project's "no silent drift" rule). The
+amendment records:
+
+- **Snapshot scope:** "before every destructive operation" →
+  "before every data/schema mutation" (single-step undo). State the
+  pedagogical rationale (intuitive Ctrl-Z; the playground favours
+  clarity over micro-optimisation).
+- **Confirmation rationale:** the original "undo is rare and
+  consequential, so always confirm" is replaced by "undo is
+  single-step and confirmation names the exact command being undone."
+  Confirmation is still always shown; there is still no suppress flag.
+  The summary simplifies — with per-command snapshots, undo rolls back
+  exactly one command, so the "discarded changes" summary is just that
+  command's description (no expensive db-diff needed). This resolves
+  ADR-0006's "counts of rows added/modified/deleted per table since
+  the snapshot" clause: under single-step undo there is no intervening
+  un-snapshotted work, so the clause collapses to "the one command."
+- **Snapshot mechanism:** clarify the hybrid (db via backup API +
+  text via copy) and how it reconciles with ADR-0015 (db is derived;
+  text authoritative) — restore re-establishes the whole triple.
+- **Storage & lifetime:** persisted ring under the project, cap 50,
+  git-ignored, export-excluded, temp-cleanup-aware.
+- **Redo:** discarded on new work.
+- **Disable switch:** `--no-undo` as a hardware escape hatch.
+- **Batch granularity:** a single user command that runs many
+  sub-operations (`replay`, future batch ops) is one undo step, taken
+  at the batch boundary. Consistent with "one undo step per user
+  command." `import` is a project switch and takes no undo step.
+
+---
+
+## 5. Architecture & design
+
+### 5.1 On-disk layout
+
+```
+<project>/
+  project.yaml
+  data/<table>.csv
+  playground.db
+  .snapshots/
+    index.json          # ordered ring + redo stack: ids, timestamps,
+                        # command text, status; the source of truth
+                        # for ordering/eviction
+    0001/
+      playground.db     # backup-API copy of the pre-op db
+      project.yaml      # pre-op copy
+      data/<table>.csv  # pre-op copies
+    0002/ ...
+    .staging/           # temp area for an in-flight snapshot
+```
+
+- **`.snapshots/` is git-ignored, export-excluded, and on the
+  temp-cleanup allowlist** (R13).
+- The **ring** (undo) and the **redo stack** are both recorded in
+  `index.json`; snapshot payload dirs are shared storage referenced by
+  id. Eviction beyond 50 deletes the oldest payload dir.
+- Ids are monotonic; never reused, to avoid stale references.
+
+### 5.2 Snapshot mechanism
+
+- **db copy:** `rusqlite::backup` — `Backup::new(&live, &snap)` copies
+  live → snapshot file; for restore, `Backup::new(&snap, &live)` copies
+  snapshot → live (works on the open worker connection; requires the
+  `backup` cargo feature, R14). No transaction may be open on the live
+  connection during a backup step.
+- **text copy:** plain file copy of `project.yaml` + every
+  `data/*.csv` into the snapshot dir. (CSVs are produced by the
+  project's hand-rolled writer that preserves NULL-vs-empty; we copy
+  the already-written files verbatim, so the distinction is preserved
+  for free — R10.)
+
+### 5.3 Timing vs ADR-0015 §6 (the load-bearing invariant)
+
+A snapshot brackets the existing 4-step persistence sequence:
+
+```
+0. STAGE snapshot  (NEW)  — backup live db + copy current text into
+                            .snapshots/.staging/. The current on-disk
+                            state is the last committed state (db, yaml,
+                            csv are committed together per ADR-0015), so
+                            staging captures the true pre-op state.
+1. open txn; validate; mutate db + metadata          (ADR-0015 step 1)
+2. write text temp files; fsync                        (step 2)
+3. atomic-rename text into place                       (step 3)
+4. commit db                                           (step 4)
+5. FINALIZE snapshot (NEW) — atomic-rename .staging/ → .snapshots/<id>/,
+                            append to index.json ring, evict oldest if
+                            >50, clear redo stack.
+   On any failure in 1–4 → txn rolls back; DISCARD .staging/.
+```
+
+This **preserves commit-db-last**: the snapshot is finalised *after*
+the db commit, and a rolled-back op leaves no ring entry (R3). A crash
+mid-sequence leaves a `.staging/` orphan, cleaned on next open exactly
+like ADR-0015's orphan text temps.
+
+### 5.4 Worker chokepoint (single point, not per-handler)
+
+Intercept centrally in `db.rs::handle_request` (the dispatcher), not
+in each `do_*` handler:
+
+- **Classify** each `Request` as *mutating* or *read-only* (a single
+  `is_mutating(&Request) -> bool`, exhaustively matched so a new
+  request variant forces a classification decision at compile time).
+- For mutating requests, **wrap** the handler call:
+  `stage_snapshot()` → run handler → on `Ok` `finalize_snapshot()` +
+  clear redo; on `Err` `discard_staging()`.
+- The command text for the snapshot meta comes from the request's
+  existing `source: String` field (added during the ADR-0034 journal
+  work).
+- **Refactor note:** mutating handlers currently send their reply via
+  the `oneshot` inside the match arm. To let the wrapper observe
+  success/failure, mutating handlers return their `Result` to the
+  dispatcher, which sends the reply **and** drives snapshot
+  finalize/discard. This is the main structural change in `db.rs`;
+  read-only handlers are untouched. (`RebuildFromText` is mutating and
+  snapshots like the rest; it reconstructs from text, but the pre-op
+  db+text are still the correct undo target.)
+- **Gate:** when undo is disabled (R9), `is_mutating` still holds but
+  the stage/finalize calls are no-ops — the worker is told at
+  construction whether snapshotting is enabled.
+
+### 5.5 Undo / redo flow (commands → modal → worker)
+
+Mirrors the existing two-phase `rebuild` modal flow, with the
+**prepare** step reading snapshot metadata rather than the project:
+
+1. `undo` parses to `Command::App(AppCommand::Undo)` →
+   `dispatch_app_command` returns `Action::PrepareUndo`.
+2. Runtime handles `PrepareUndo`: reads `.snapshots/index.json` top
+   entry (command text + timestamp) — a cheap file read, like
+   `summarize_project` does for rebuild — and posts
+   `AppEvent::UndoPrepared { command, when }` (or
+   `UndoUnavailable` if the ring is empty → friendly note, R8).
+3. `App::update` opens `Modal::UndoConfirm` showing "Undo
+   `<command>` (run <relative time>)? [Y] yes  [N] no". `Y` →
+   `Action::Undo`; `N`/`Esc` → close + "undo cancelled".
+4. Runtime handles `Action::Undo` → worker `Request::Undo`:
+   - push a snapshot of **current** state onto the redo stack,
+   - restore the top ring snapshot (backup-restore db + rename text
+     back), pop the ring,
+   - reply with the undone command text.
+5. Runtime posts `AppEvent::UndoSucceeded { command }` /
+   `UndoFailed { error }`; `App` closes the modal, notes the outcome,
+   and triggers a re-query so the view refreshes (R11).
+
+`redo` is symmetric (`PrepareRedo` / `Redo`, `Request::Redo`, restores
+the top redo entry, pushes current onto the undo ring). Whether redo
+also confirms is an open sub-decision (§10).
+
+### 5.6 Batch operations — one undo step (Begin/EndBatch)
+
+`run_replay` (`runtime.rs:1622`) dispatches **one worker request per
+replayed command** via `execute_command_typed`. Left to §5.4, a long
+replay would stage one snapshot per line. Instead, a reusable batch
+primitive collapses a batch to a single ring entry:
+
+- `Request::BeginBatch { source }` — stages **one** snapshot (meta =
+  the batch command, e.g. `replay history.log`) and sets the worker's
+  `in_batch` flag. While set, per-request staging in §5.4 is **skipped**
+  (the boundary snapshot already captured pre-batch state).
+- `Request::EndBatch` — finalizes the staged snapshot into the ring
+  **only if ≥1 mutation occurred** during the batch (else discards it,
+  so an all-skips replay leaves no no-op undo step); clears `in_batch`
+  and the redo stack.
+- `run_replay` brackets its loop with begin/end. A mid-replay
+  `PersistenceFatal` tears the runtime down before `EndBatch`, leaving
+  a `.staging/` orphan that is cleaned on next open (ADR-0015 parity);
+  undo state is moot at that point anyway.
+- The same primitive serves any future in-project batch command.
+- `import` does **not** use this — it creates a new project and
+  switches (ADR-0015 §11); the current project is untouched, so there
+  is nothing to snapshot and no undo step. `rebuild` is already a
+  single request (`RebuildFromText`) → naturally one snapshot, no
+  batch handling needed.
+
+### 5.7 New / changed types (from the Explore survey)
+
+| Layer | Change |
+|---|---|
+| `Cargo.toml` | add `"backup"` to `rusqlite` features |
+| `src/cli.rs` | `Args.no_undo: bool`; parse `--no-undo` |
+| `src/dsl/command.rs` | `AppCommand::Undo`, `AppCommand::Redo` |
+| `src/dsl/grammar/app.rs` | `UNDO` / `REDO` `CommandNode` (EMPTY_SEQ, like `REBUILD`) |
+| `src/action.rs` | `PrepareUndo`, `Undo`, `PrepareRedo`, `Redo` |
+| `src/event.rs` | `UndoPrepared`/`UndoUnavailable`/`UndoSucceeded`/`UndoFailed` (+ redo) |
+| `src/app.rs` | `Modal::UndoConfirm` (+ redo) struct; event arms; `handle_undo_confirm_key`; dispatch arms |
+| `src/ui.rs` | `render_undo_confirm` (mirror `render_rebuild_confirm`) |
+| `src/db.rs` | `is_mutating`; `stage/finalize/discard_snapshot`; `Request::Undo`/`Redo`/`BeginBatch`/`EndBatch`; dispatcher wrap; `undo_enabled` + `in_batch` worker fields |
+| `src/persistence/` (or new `src/snapshots/`) | ring + redo store, `index.json`, eviction, restore |
+| `src/runtime.rs` | `is_app_lifecycle_entry_word += undo,redo`; `PrepareUndo/Undo` (+redo) handling; thread `no_undo` to worker; bracket `run_replay` with Begin/EndBatch |
+| `src/project/mod.rs` | `.snapshots/` in `.gitignore` template |
+| `src/archive.rs` | exclude `.snapshots/` from export |
+| `safely_delete_temp_project` | `.snapshots/` on the contents allowlist |
+| `src/completion.rs` | `undo`/`redo` in app-command entry keywords |
+| `src/friendly/strings/en-US.yaml` | modal title/prompt, success/cancel/disabled notes, CLI help |
+
+---
+
+## 6. Phase 2/3 — candidates considered (condensed)
+
+The mechanism choice was the crux; three candidates were evaluated and
+the user selected the hybrid (§2.1). Recorded here so the rejected
+options aren't silently dropped:
+
+| Candidate | Stores | Undo restore | Verdict |
+|---|---|---|---|
+| **C1** db-only via backup API | db | restore db + re-derive text via `finalize_persistence` | Honors ADR-0006 letter; needs a new "db→all-text" re-derive path. Rejected: less direct restore. |
+| **C2 (chosen)** hybrid | db + yaml + csv | restore all three directly | Most robust/direct restore; satisfies ADR-0006 + ADR-0015 with no mechanism amendment. **Selected.** |
+| **C3** text-only | yaml + csv | restore text + `rebuild_from_text` | Smallest disk; reuses tested rebuild. Rejected: rebuild dependency at undo time is less direct for a safety feature. |
+
+Scope candidates (destructive-only / +UPDATE / every-mutation) and
+redo semantics were likewise put to the user; selections in §2.
+
+---
+
+## 7. Devil's Advocate review of this plan
+
+- **Per-mutation snapshot cost.** Backing up the db + copying all CSVs
+  before *every* mutation is the real cost; a bulk paste of N inserts
+  makes N snapshots. *Mitigations:* teaching-scale dbs are small;
+  ring cap 50 bounds total storage; `--no-undo` is the escape hatch;
+  the ADR-0015 "batch command" remains the future remedy. A
+  hardlink/CoW optimisation for unchanged files between consecutive
+  snapshots is noted as a **future** improvement, not v1.
+- **Disk growth.** 50 × (db + text) could reach tens of MB for larger
+  projects. Honest consequence; bounded by the cap and the disable
+  flag. Flagged in the amendment's Consequences.
+- **Backup API + open transaction.** A backup step requires no open
+  txn on the live connection. The stage step runs *before* the
+  mutation txn opens, and restore runs outside any txn — safe by
+  construction, but tests must assert it (R20).
+- **Restore must include internal metadata tables.** The db backup is
+  byte-faithful so `__rdbms_playground_*` tables come along
+  automatically; the text copy carries the yaml/csv. R10 test guards
+  this explicitly.
+- **Temp-cleanup interaction.** A mutation in a fresh temp creates
+  `.snapshots/`; if the user then undoes back to empty, an empty temp
+  may carry a `.snapshots/` dir. The allowlist entry (R13) keeps
+  `safely_delete_temp_project` willing to delete it; a test covers it.
+- **Replay safety.** Without R12, a `history.log` containing `undo`
+  would abort/panic replay (ADR-0034). Adding to both the runtime
+  filter and the completion set is mandatory, not optional.
+- **`is_mutating` exhaustiveness.** Must be a non-`_` match so any
+  future `Request` variant fails to compile until classified —
+  prevents a new mutation silently escaping the snapshot hook.
+
+No DA finding is deferred; all are addressed above or tracked as
+explicit future items (hardlink optimisation) for user awareness.
+
+---
+
+## 8. Implementation sequence (test-first throughout)
+
+1. **Cargo + CLI:** add `backup` feature; `--no-undo` flag + parse
+   tests. (R14, R9-partial)
+2. **Snapshot store module:** `index.json` model, ring + redo, stage/
+   finalize/discard/restore, eviction — Tier-1 tests first against a
+   temp dir + in-memory/temp db. (R2, R3, R4, R6, R7)
+3. **Worker integration:** `is_mutating` (exhaustive), dispatcher wrap,
+   `undo_enabled` gate, `Request::Undo`/`Redo`. Tier-3 tests:
+   mutation→snapshot, undo restores db+text, redo, eviction,
+   redo-clear, ordering preserved, `--no-undo` no-ops. (R1, R3, R10)
+4. **Commands + grammar:** `AppCommand::Undo/Redo`, grammar nodes,
+   parse tests; replay-filter + completion entries + their lockstep
+   test. (R5, R7, R12)
+5. **Action/event/modal/UI:** prepare→confirm→execute flow; modal key
+   handling (Tier 1); modal + disabled-note rendering (Tier 2 insta);
+   re-query on success. (R5, R8, R11, R19)
+6. **Cross-cutting files:** `.gitignore` template, export exclusion,
+   temp-cleanup allowlist — each with a test. (R13)
+7. **Full-stack flow** mixing DSL + SQL mutations, undo/redo across
+   both, reopen-and-verify-persistence. (R21, R22)
+8. **Docs:** ADR-0006 amendment + README index; requirements U1/U2;
+   CLAUDE.md line. (R16, R17)
+9. **Phase 5 verification:** full suite green, zero skips, no
+   regression vs §1; DA final pass. (R23)
+
+---
+
+## 9. Engine-neutral string notes (ADR-0002)
+
+User-facing text uses "snapshot", "undo", "the database". Never
+"SQLite", "backup API", "PRAGMA", "VACUUM". Modal: "Undo `<command>`
+(run <relative time>)?". Disabled: "Undo is turned off for this
+session." Empty: "Nothing to undo." These wordings are placeholders
+pending the catalog edit.
+
+---
+
+## 10. Sub-decisions — all confirmed (2026-05-24)
+
+1. **Disable mechanism shape:** `--no-undo` CLI flag only for v1 (no
+   in-app toggle).
+2. **Redo also confirms,** naming the command to re-apply (symmetry
+   with undo).
+3. **Redo is in scope** for this pass (discard-on-new-work semantics).
+4. **Confirm key:** `Y` confirms / `N` / `Esc` cancel, matching the
+   rebuild modal.
+
+**Still open (one question to the user):** whether project-switch-level
+"undo back to the previous project" should be tracked as a separate
+future feature. Default if unanswered: leave `import`/project-switch
+wholly outside undo (the prior project is intact on disk and reachable
+via `load` / `--resume`).
+
+---
+
+## 11. Next action after approval
+
+On sign-off: write the ADR-0006 amendment first (it's the contract),
+update the README index, then implement per §8 — test-first, escalate
+any further ADR-vs-implementation mismatch rather than deciding.

docs/requirements.md

@@ -365,11 +365,23 @@ handoff-14 cleanup; 449 after B2/C2.)
 
 ## Undo and replay (per ADR-0006)
 
-- [ ] **U1** Auto-snapshot before destructive operations into a
-  ring buffer (initial size N=10, tunable).
-- [ ] **U2** `undo` restores the most recent snapshot; `redo`
-  re-applies; both prompt for confirmation showing the snapshot
-  timestamp and a summary of the changes that will be discarded.
+- [ ] **U1** Auto-snapshot before **every** data/schema mutation
+  (DSL + SQL) into a persisted ring buffer (size N=50, tunable),
+  per ADR-0006 Amendment 1 (single-step undo, superseding the
+  original destructive-only model). Snapshot is a hybrid
+  whole-project copy (database via online backup API + `project.yaml`
+  / `data/*.csv` as files); staged before the mutation's transaction,
+  finalised after the db commit (preserves ADR-0015 §6). A batch
+  command (`replay` / future batch ops) records one boundary
+  snapshot; `import` takes none. A `--no-undo` CLI flag disables
+  snapshotting. *(Designed + amended + planned in
+  `docs/plans/20260524-adr-0006-undo-snapshots.md`; not yet
+  implemented.)*
+- [ ] **U2** `undo` restores the most recent snapshot (database +
+  text, directly); `redo` re-applies (redo stack discarded on new
+  work); both prompt for confirmation naming the command being
+  undone / re-applied (`Y` confirms). *(Designed + amended; not yet
+  implemented — see U1's plan.)*
 - [x] **U3** `history.log` records every submitted command in
   append-only form, tagged with its outcome (Iteration 2;
   broadened by ADR-0034). Format: `<ISO-8601 Z>|<status>|<source>`