docs(plan): H2 contextual hint implementation plan (ADR-0053)

Phased build plan: mechanism skeleton with tier-2 fallback first
(hint_id field, AppCommand::Hint, F1 read-only overlay, last_error_hint_key,
note_hint* renderer), then catalogue + the three approved exemplars,
then comprehensive content in batches, then polish. Reuses the existing
command_for_entry_word / usage_keys_for_input_in_mode lookups for
command identification. Test spine includes the comprehensiveness
coverage test that gates "comprehensive for v1".

docs/plans/20260614-adr-0053-contextual-hint-H2.md

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+# Plan — ADR-0053: contextual `hint` command + F1 keybinding (H2)
+
+Implements ADR-0053. Closes the last open piece of **A1** (the canonical
+app-command set) and requirements **H2**. No Gitea issue — this is
+requirements-driven work; any genuine "later" item found en route gets
+its own issue (cf. #36, already filed for the parallel `help`-side gap).
+
+## 1. Goal
+
+Give learners on-demand, **teaching-grade** contextual help — a *third*
+tier beneath the existing terse always-on text (tier 1) and the
+short contextual lines that are already shown (tier 2: the live ambient
+prose, and the error `hint:` which is on by default since
+`Verbosity::Verbose` is the default). Two surfaces:
+
+- **F1** (read-only overlay) → a tier-3 block for the **live partial
+  input**, or — on empty input — for the **most recent runtime error**.
+- **`hint`** (submitted app command) → the tier-3 block for the **most
+  recent runtime error** (the buffer is empty post-submit, so it can only
+  act on recent context).
+
+The mechanism is small; the **content corpus is the feature** (~80
+blocks, comprehensive for v1, authored exemplars-first per ADR-0053 D7).
+
+## 2. The shape of the work (why this order)
+
+The mechanism and the content are separable, and the mechanism should
+land first with **graceful tier-2 fallback** so every surface works
+before any tier-3 text exists. That lets us:
+
+- build + test the trigger matrix / routing / `:`-strip / read-only-
+  overlay behaviour against a skeleton (TDD), then
+- pour in content in reviewable batches without re-touching the wiring,
+- and turn on the **comprehensiveness coverage test** only once the
+  corpus is complete (it is red until then — by design).
+
+Build order: **Phase A** (mechanism skeleton, falls back to tier-2) →
+**Phase B** (catalogue structure + the three approved exemplars) →
+**Phase C** (comprehensive content, batched) → **Phase D** (polish:
+strip advertisement, snapshots, full green).
+
+## 3. Grammar: the `hint_id` field + the `HINT` node
+
+### 3a. New `CommandNode.hint_id`
+- Add `pub hint_id: Option<&'static str>` to `CommandNode`
+  (`src/dsl/grammar/mod.rs:512`, beside `help_id` / `usage_ids`), with a
+  doc comment mirroring `help_id`'s. Compiler will force every node
+  literal (~37, across `grammar/app.rs`, `data.rs`, `ddl.rs`) to set it —
+  in Phase A set them all to `None` (everything falls back to tier-2);
+  fill them in Phase C.
+- **Why `hint_id` not `help_id`** (ADR-0053 D3): `help_id` is `None` on the
+  7 advanced-SQL forms purely to dedup the `help` *list*; those forms have
+  distinct SQL syntax and need their own block. `hint_id` is 1:1 with
+  forms.
+
+### 3b. `AppCommand::Hint` + the `HINT` node
+- `AppCommand::Hint` variant (no fields — no topic arg) in
+  `src/dsl/command.rs:544`.
+- `pub static HINT: CommandNode` in `grammar/app.rs` mirroring `HELP` but
+  with **no topic shape** (bare keyword, like `UNDO`): `entry:
+  Word::keyword("hint")`, `shape: EMPTY_SEQ` (as `UNDO`,
+  `grammar/app.rs:333`), `ast_builder:
+  build_hint` (returns `Command::App(AppCommand::Hint)`), `help_id:
+  Some("app.hint")`, `hint_id: Some("app.hint")`, `usage_ids:
+  &["parse.usage.hint"]`.
+- Register `(&app::HINT, CommandCategory::Simple)` in `REGISTRY`
+  (`grammar/mod.rs`), beside `HELP`. (App commands are available in both
+  modes via the existing mechanism.)
+
+## 4. Command identification (live-input → node)
+
+The F1 live-input path needs "which command form is being typed." **The
+lookup machinery already exists** — do not rebuild entry matching:
+- `command_for_entry_word(word) -> Option<(usize, &'static CommandNode)>`
+  (`grammar/mod.rs:811`) returns the matched node for an entry word
+  (Simple-first; the caller extracts the first word of the input).
+- `usage_keys_for_input_in_mode(source, mode)` (`grammar/mod.rs:564`)
+  already performs the **mode-aware** Simple/Advanced selection the hint
+  path needs (advanced `create` → the SQL nodes, simple → the DSL node) —
+  it just returns `usage_ids` rather than the node.
+- **The only new bit:** a thin `hint_id_for_input_in_mode(source, mode)`
+  (or a node-returning sibling of `usage_keys_for_input_in_mode`) that
+  applies the same mode selection and returns the chosen node's
+  `hint_id`. Mirror the existing function; don't duplicate its matching.
+- **`:`-strip:** in Simple mode, strip a leading `:` (one-shot escape,
+  ADR-0003) before identification so `: SELECT …` resolves to the
+  advanced `SELECT` node.
+- No match (empty / unrecognised entry word) → the "getting started"
+  pointer (D2).
+
+## 5. F1 keybinding (read-only overlay)
+
+In `App::handle_key` (`src/app.rs:1155`):
+- Add an F1 arm (`KeyCode::F(1)`) **after** the modal gate and the
+  sidebar-nav gate (inert there, per D2), and **before** the
+  "any other key clears the completion memo" fall-through (`_ =>
+  self.last_completion = None`, ~line 1228) — F1 must **not** clear the
+  memo or touch the buffer/cursor (D1).
+- Behaviour (the trigger matrix, D2):
+  - non-empty input → `note_hint_for_input()` (the command's `hint.cmd`
+    block + the live "Next:" expected-set from the walker).
+  - empty input + `last_error_hint_key` set → `note_hint_for_error()`.
+  - empty input + no recent error → `note_getting_started()`.
+- Returns `Vec::new()` (pure output emission, like `help`).
+- `demo_badge_label` (`app.rs:520`) gains an `F1 → "[F1]"` entry so demo
+  mode surfaces it (ADR-0047).
+
+## 6. The two error routes (D2 / D5)
+
+- **Runtime errors:** add `last_error_hint_key: Option<String>` to `App`.
+  Set it where friendly errors are rendered (`runtime.rs:2615`,
+  `app.rs:2424`) from the error's class key; clear on the next successful
+  command. The `hint` command and empty-input F1 read it.
+- **Pre-submit diagnostics:** the F1 live-input path, when the input
+  carries an under-cursor diagnostic, reads it straight from the walker
+  (`input_diagnostics_in_mode`, the same source the ambient panel uses)
+  and renders that diagnostic's `hint.err.<class>` block instead of (or
+  alongside) the command block. No stored state.
+- Both render from `hint.err.*`.
+
+## 7. Rendering: the `note_hint*` family (D4)
+
+- New `App::note_hint_for_input`, `note_hint_for_error`,
+  `note_getting_started` (siblings of `note_help`/`note_help_topic`,
+  `app.rs:2982`/`3021`).
+- A tier-3 block is **structured** (`what` / `example` / `concept`, plus
+  the live `Next:` line on the input path). The catalogue stores each part
+  under sub-keys (`hint.cmd.<id>.what`, `.example`, `.concept`); the
+  renderer fetches each via `t!` and lays them out as a small framed
+  block.
+- Styling: `OutputKind::System`; `OutputStyleClass::Hint` (muted) on
+  `what`/`concept`/`Next`, `Neutral` on `example` so the runnable line
+  stands out. Reuse `OutputLine::styled` + `push_category_three_prose`
+  patterns (`app.rs:3121`).
+- **Fallback:** if a node's `hint_id` is `None` or a key is missing,
+  degrade to tier-2 (ambient prose for the input path; the verbose error
+  `hint:` for the error path) — never blank.
+
+## 8. Catalogue + `keys.rs`
+
+- New sub-namespaces under the existing top-level `hint:` in
+  `src/friendly/strings/en-US.yaml`: `hint.cmd.<hint_id>.{what,example,
+  concept}` and `hint.err.<class>.{what,example,concept}`.
+- Register every key + its placeholders in `src/friendly/keys.rs`
+  (`KEYS_AND_PLACEHOLDERS`) so the build-time validation covers them.
+- `parse.usage.hint` + `help.app.hint` strings for the command itself.
+
+## 9. Content (Phase C — the bulk, batched per D7)
+
+Exemplars approved in the ADR (`insert` live-input, FK child-side error,
+`add relationship`) are the template. Author in reviewable batches:
+1. **App commands** (~16): save/save as/load/new/rebuild/export/import/
+   replay/undo/redo/mode/messages/copy/help/hint/quit.
+2. **DDL** (simple): create table, create m:n, add column/relationship/
+   index, drop, rename, change column.
+3. **DML** (simple): insert, update, delete, show, seed, explain,
+   select/with.
+4. **Advanced-mode SQL forms** (7): SQL CREATE TABLE, ALTER TABLE,
+   CREATE/DROP INDEX, DROP TABLE, SQL INSERT/UPDATE/DELETE, EXPLAIN SQL —
+   **own blocks, SQL-syntax examples**.
+5. **Runtime error classes** (9): unique, foreign_key ×{child,parent},
+   not_null, check, type_mismatch, not_found, already_exists, generic,
+   invalid_value.
+6. **`diagnostic.*` classes** (~33): arity/type/unknown-table-column/etc.
+
+Each block: `what` (1–2 sentences), `example` (one runnable line,
+mode-correct), `concept` (the relational idea — the teaching part;
+optional only where genuinely none, e.g. `quit`).
+
+## 10. Tests
+
+Written test-first against the Phase-A skeleton where possible.
+
+- **Tier 1 (unit, `app.rs`):**
+  - trigger matrix: F1 non-empty → command block; F1 empty + recent error
+    → error block; F1 empty + none → getting-started; `hint` command +
+    error → error block; `hint` + none → getting-started.
+  - `last_error_hint_key` set on a failing command, cleared on the next
+    success.
+  - routing: a pre-submit diagnostic on the input drives the diagnostic
+    `hint.err`; a runtime error drives the stored-key route.
+  - `:`-strip: `: SELECT …` in Simple mode resolves to the advanced node.
+  - **read-only overlay:** F1 leaves `input`, `input_cursor`, and
+    `last_completion` unchanged.
+  - tier-2 fallback when `hint_id`/key absent.
+- **Tier 2 (`insta`):** snapshot a representative rendered tier-3 block
+  (the `insert` exemplar) so the framed layout + styling spans are locked.
+- **Tier 3 (integration, `tests/it/`):** type a partial command → F1 →
+  block appears, buffer untouched; run a failing insert → `hint` → FK
+  error expansion.
+- **Comprehensiveness coverage test** (enforces D6, the key one): iterate
+  `REGISTRY` and assert every node has a `hint_id` resolving to a
+  `hint.cmd.*` block; assert every runtime-error + `diagnostic.*` class
+  has a `hint.err.*` block. **Red until Phase C completes** — enable
+  (un-`ignore`) as the final gate.
+- `keys.rs` validation continues to guarantee every *referenced* key
+  resolves.
+
+## 11. Keybinding strip + discoverability (Phase D)
+
+- The ADR-0051 bottom strip advertises **F1 = hint** in the editing/
+  typing state (and on the empty-input state, since F1 still does
+  something there). Re-accept the affected full-panel snapshots.
+
+## 12. ADR / docs
+
+- ADR-0053 is committed (`e16ad50`). On completion, flip its Status from
+  "implementation pending" to implemented (with date), and update the
+  README index entry + `requirements.md` **H2 → [x]** and **A1 → [x]**
+  (A1 closes when `hint` lands).
+
+## 13. Risks / watch-list
+
+- **Command-identification reuse.** The lookup exists
+  (`command_for_entry_word` + the mode-aware `usage_keys_for_input_in_mode`,
+  `grammar/mod.rs:811`/`564`); the only new code is a thin node/`hint_id`
+  variant that reuses their selection. Do **not** re-implement entry-word
+  matching — mirror the existing functions.
+- **Structured-key ergonomics.** Three sub-keys per block × ~80 blocks is
+  ~240 catalogue keys; keep the `keys.rs` registration generation tidy
+  (consider a helper that registers the `{what,example,concept}` triple
+  for an id).
+- **Content voice drift across batches.** Re-check each batch against the
+  approved exemplars; the `concept` line is where drift (too terse / too
+  advanced) creeps in. Pedagogy wins ties.
+- **F1 terminal capture.** A few terminals intercept F1; acceptable
+  (it's the convention) but note it if testing surfaces it.
+- **Snapshot churn.** The strip change re-accepts ADR-0051 snapshots;
+  keep that diff isolated.
+- **Coverage-test timing.** It is red through Phases A–C; gate it so CI
+  isn't broken mid-stream (e.g. `#[ignore]` until the final batch), then
+  make passing it the completion criterion.
+```