docs: ADR-0042 — continue H1a parse-error pedagogy on the grammar tree

ADR-0020/0021 specified a chumsky-based H1a; ADR-0024 replaced chumsky
with the scannerless walker, leaving both obsolete. Mark them superseded
(kept as institutional memory) and add ADR-0042, which restates H1a
against the architecture as built.

ADR-0042 records that H1a is substantially shipped already — per-command
usage block, available-commands fallback, source-derived ident slot
labels, curated parse.custom.* near-miss messages, and schema-aware
[ERR] diagnostics — and defines the remaining work: a verified
per-command near-miss matrix (the definition of done), friendlier
literal expectation labels that add role context while keeping the
exact literal visible, and advanced-mode SQL parse parity (RETURNING
scope, CROSS JOIN ON, INSERT…SELECT count), kept distinct from
ADR-0019 §OOS-2 engine-error sanitisation.

- docs/adr/0020,0021: superseded notes + README entries
- docs/adr/0042: new ADR
- docs/adr/README.md: index upkeep (ADR-0000 rule)

docs/adr/README.md

@@ -25,8 +25,8 @@ This directory contains the project's ADRs, recorded per
 - [ADR-0017 — Column type-change compatibility](0017-column-type-change-compatibility.md)
 - [ADR-0018 — Auto-fill contracts for `serial` and `shortid` columns](0018-auto-fill-contracts-for-serial-and-shortid.md)
 - [ADR-0019 — Friendly error layer (H1) and i18n message catalog](0019-friendly-error-layer-and-i18n.md)
-- [ADR-0020 — Tokenization layer for the DSL parser](0020-tokenization-layer-for-the-dsl-parser.md)
-- [ADR-0021 — Parser-as-source-of-truth for H1a (per-command usage in parse errors)](0021-parser-as-source-of-truth-for-h1a.md)
+- [ADR-0020 — Tokenization layer for the DSL parser](0020-tokenization-layer-for-the-dsl-parser.md) — **Superseded by ADR-0024 (never implemented).** Specified a `chumsky`-over-tokens architecture (separate lexer, `define_keywords!`, `&[Token]` grammar). ADR-0024 adopted a scannerless hand-rolled walker and removed `chumsky` entirely; the lexer/keyword/token model here does not exist. Kept as institutional memory of the path not taken.
+- [ADR-0021 — Parser-as-source-of-truth for H1a (per-command usage in parse errors)](0021-parser-as-source-of-truth-for-h1a.md) — **Mechanism superseded by ADR-0024; H1a scope continued in ADR-0042.** The *intent* (show the command's grammar at the point of error) shipped — `usage_ids` on each `CommandNode`, the `parse.usage.*` templates, and the `available_commands` fallback all exist — but via grammar nodes, not the `chumsky` `UsageEntry` registry / `parse.token.*` keys described here (which were never built).
 - [ADR-0022 — Ambient typing assistance: colour, hint panel, completion (I3 + I4)](0022-ambient-typing-assistance.md) — **Amendment 1 supersedes §12's simple-mode-only carve-out**: the unified mode-aware walker (ADR-0030/0031/0032) now speaks SQL, so advanced-mode ambient assistance is re-enabled. `ambient_hint_in_mode` + `hint_resolution_at_input_in_mode` + `expected_for_hint_snapshot` thread `Mode`; `render_hint_panel` calls ambient for all modes (no more advanced-mode `None`); the one-shot `:` sigil is stripped before the ambient walk. Fixes a live bug where advanced-mode SQL hinting/completion-preview were dead despite Phase 2 marking them green (validated at the engine layer, not the UI). Simple-mode gating, highlighting, and the §13 performance posture are unchanged; covered by an app-level render test plus ambient-layer regression locks; **Amendment 2 reverses the handoff-14 keywords-first candidate ordering** — schema identifiers (table/column/relationship names) now sort *before* keywords so a name the user would have to look up stays visible in the single-row, window-scrolled candidate line (keywords are learned over time; the `tok_identifier`/`tok_keyword` colour split marks the boundary); shipped with a `walk_repeated` fix that surfaces a list item's trailing optionals at a clean boundary (`order by Name ` → `asc`/`desc`, `select Name ` → `as`, `create table … Code(text) ` → `not`/`unique`/`default`/`check`; the `,` separator deliberately not surfaced); records a deferred two-line hint box for growing lists; **Amendment 3 makes the ambient-hint fallback rung schema-aware** — Amendment 1's bottom-rung `parse_command_in_mode` was schemaless while every earlier rung was not, so between-values insert hints pointed at `)` (type-blind close) instead of `,` and wrong-arity closed tuples read "submit with Enter" for an input the schema-aware parse rejects (issue #2); now uses `parse_command_with_schema_in_mode`, no extra walk, with the friendly arity diagnostic still winning at its higher rung; **Amendment 4 gives column types a dedicated highlight class** — both `Node::Ident.highlight_override` *and* the `Word.highlight_override` field were dead (driver destructured the former to `_`, `walk_word` hardcoded `Keyword`); now both wired through, with a new `HighlightClass::Type` + eighth `Theme` field `tok_type` (a pink/deep-magenta distinct from both keyword purple and identifier teal) so types no longer render identically to identifiers (issue #8); the three `IdentSource::Types` slots opt in via `Some(Type)` (advanced-mode single-word SQL aliases — `float`, `varchar`, … per ADR-0035 §3 — ride along for free), and the two-word `double precision` alias opts in via the new `Word::type_keyword` constructor so it matches its synonyms; **Amendment 5 lets the hint panel grow for long prose hints** — a fixed one-row panel clipped long field-value/usage hints past the first line (issue #12); `resolve_hint_lines` now pre-wraps prose and `render_right_column` sizes the panel to the line count (1 row default, up to `MAX_HINT_ROWS`=3, reclaimed when short) with a `clamp_wrapped` ellipsis backstop; the candidate list still scrolls horizontally on one row (Amendment 2's deferred two-line candidate box stays deferred); also shortens the 299-char `parse.usage.sql_create_table` synopsis to a terse one-liner (full grammar remains in `help.ddl.sql_create_table`); **Amendment 6 adds a curated SQL function-name list** (`src/dsl/sql_functions.rs`, `KNOWN_SQL_FUNCTIONS` — aggregates + common + broader scalars; `cast` deliberately excluded as its `CAST(x AS type)` syntax isn't a plain-call shape) as the single source of truth shared by two consumers at the `sql_expr_ident` slot (ADR-0031 §1): **issue #15** offers the functions as Tab candidates under a new `CandidateKind::Function` + ninth `Theme` colour `tok_function` (a blue distinct from keyword/identifier/type, parallel to Amendment 4's `tok_type`) so a learner discovers `sum`/`upper`/…; **issue #16** restores the typing-time column-typo flag the issue-#6 fix had dropped wholesale at this slot — `invalid_ident_at_cursor` now bails only when the partial prefix-matches a known function, else falls through to the schema-column check, so `select Agx` warns again at typing time while `select sum` does not (the issue-#6 lockdown tests + the submit-time `unknown_column` diagnostic path are untouched, and the no-validation-allowlist posture stands); see ADR-0031's status note for the grammar-side anchor
 - [ADR-0023 — Unified declarative grammar tree](0023-unified-grammar-tree.md) — direction (superseded for execution detail by ADR-0024)
 - [ADR-0024 — Unified grammar tree: execution plan](0024-unified-grammar-tree-execution-plan.md) — **Accepted**, the executable spec — implemented (Phases A–F; Phase F shipped "minimal", `parser.rs` retained as the router — see the ADR's Phase F implementation note)
@@ -47,3 +47,4 @@ This directory contains the project's ADRs, recorded per
 - [ADR-0039 — EXPLAIN over advanced-mode SQL queries](0039-explain-over-advanced-sql.md) — **Accepted** (2026-05-27), **implemented 2026-05-30 (issue #7)**, **supersedes ADR-0030 §13 OOS-2**. Lets `explain` wrap the advanced SQL commands (`Select`/`SqlInsert`/`SqlUpdate`/`SqlDelete`, plus `with`/CTE which builds a `Select`) in addition to the DSL `ShowData`/`Update`/`Delete` it already covers (ADR-0028), running `EXPLAIN QUERY PLAN` over the validated SQL text through the existing ADR-0028 span-styled plan tree (advanced mode only; DSL `explain` unchanged in both modes). Implemented via a second `Advanced` `explain` CommandNode (`EXPLAIN_SQL`) registered under the shared `explain` entry word — reusing the established `insert`/`update`/`delete` shared-word dispatch (`decide`: SQL-first / DSL-fallback), so `explain show data …` and DSL-only `--all-rows` still reach the DSL node; rejected a `DynamicSubgrammar` mode-gate (its resolution cache key omits `mode`). `build_explain_sql` slices the inner SQL off the source (excludes `explain`) and reuses the existing SQL builders; `do_explain_plan` runs the carried text verbatim, no params. Advanced `explain update`/`delete` now route through SQL (identical plan, full SQL syntax); DSL-explain tests pinned to simple mode. Reframed OOS-2 as a *deferred* exclusion (per ADR-0000's out-of-scope discipline), not a rejection. OOS (deferred): EXPLAIN of DDL (no query plan exists)
 - [ADR-0040 — A per-command completion marker (✓/✗) replaces the `[ok]` summary line](0040-completion-marker-replaces-ok-summary.md) — **Accepted 2026-05-30 (issue #9)**, amends ADR-0014 / ADR-0028 / ADR-0019 output conventions, builds on ADR-0037's mode-tagged echo. An audit of the whole command surface found the `[ok] <verb> <subject>` summary line duplicates the echo line above it (verb+subject) everywhere; its only unique contribution is the success-vs-error signal (and `explain select` even rendered `[ok] explain` with an empty subject post-ADR-0039). Decision: drop the `[ok]` line and the symmetric `"…" failed:` prefix; the echo line gains a trailing inline **✓** (green, success) / **✗** (red, failure) — `running:` becomes a pending state that resolves to `<input> ✓/✗` on completion (status set via the existing `rfind(Echo)` lookup). Content (row counts, structure, data, plan tree, teaching echo) unchanged. Scoped to the DSL/data/SQL family that has the redundant echo+`[ok]` pair; app-command `[ok]` lines (`rebuild`/`export`/`now editing`) are payload-bearing, have no echo to mark, and stay as-is. `ok.summary` retired; `dsl.failed` reduced to the rendered reason. Broad but mechanical snapshot churn. OOS: app-command `[ok]` lines, the `[WRN]` validity indicator, and the tag colours (issue #10)
 - [ADR-0041 — Copy the output panel to the system clipboard](0041-copy-output-to-clipboard.md) — **Accepted 2026-06-02 (issue #11)**, amends ADR-0003's app-command registry (adds **`copy`** / `copy all` / `copy last`). The friction it removes: filing a bug report meant terminal-selecting the output panel and fighting wrapping/borders. New **app-level command** (sigil-free, both modes): `copy` / `copy all` copy the whole panel; `copy last` copies from the most recent echo line to the end. **Mechanism — OSC 52 *and* native (`arboard`), always both**, because OSC 52 acceptance is undetectable (no terminal ack), so a true "fall back when unsupported" can't be built: emit the OSC 52 escape (no new dep — `base64`+`crossterm`; works over SSH; tmux-passthrough-wrapped via `$TMUX`), then a best-effort native write whose failure is ignored (headless host — OSC 52 carried it); the two carry identical content. **Format — plain text verbatim as rendered** (tags, `✓`/`✗`, box-drawing) joined by `\n`, without viewport padding/wrapping; a drift-lock test pins `OutputLine::plain_text` to `render_output_line`. `arboard` added **`--no-default-features`** (drops the `image` crate; X11-only on Linux — `wayland-data-control` deliberately omitted as it ~doubles the dep tree and OSC 52 covers native-Wayland). Security: write-only, scans clean for arboard's tree (cargo audit / osv-scanner / grype), 1Password-maintained, minimal surface. OOS: Markdown export, selection/range, a keybinding, OSC 52 read, `screen` passthrough
+- [ADR-0042 — H1a parse-error pedagogy in the grammar-tree era](0042-h1a-parse-error-pedagogy-grammar-tree.md) — **Accepted 2026-06-03.** Continues **H1a** from ADR-0021 against the ADR-0024 grammar tree (ADR-0021's chumsky mechanism is dead). Records the **baseline already shipped** — per-command `usage:` block (38 `parse.usage.*` templates), available-commands fallback, structural "after `…`, expected …" wording, source-derived ident slot labels ("table name"/"column name"), curated `parse.custom.*` near-miss messages, and the ADR-0027/0033/0036 schema-aware `[ERR]` diagnostics — so H1a is *substantially* delivered at the intent level. Defines the remaining work as **(1)** a verified per-command **near-miss matrix** (`tests/typing_surface/` + `tests/it/parse_error_pedagogy.rs`) as the definition of done, test-first; **(2)** **friendlier literal expectation labels** — optional prose glosses on `Word`/`Punct`/`Flag` positions that *add* role context while always keeping the exact literal visible (e.g. "a filter clause: `where …` or `--all-rows`"); **(3)** **advanced-mode SQL** near-miss parity (RETURNING scope, CTE-arity positioning, `CROSS JOIN … ON`, INSERT…SELECT count) — **in scope**, kept distinct from ADR-0019 §OOS-2 which covers advanced-SQL *engine*-error sanitisation, a different layer. Catalog/anchor-phrase discipline (ADR-0019) preserved; no public API change. OOS: I3/I4, spell-correction, multi-error reporting, verbosity-gating the usage block