ADR-0022: ambient typing assistance (unifies I3 + I4)

Replaces the originally-planned separate ADRs for syntax
highlighting (I4) and tab completion (I3) with a single
unified design. The framing: colour, hint panel, and Tab
are three answers to the same question — what does the user
need to know mid-typing? — and planning them separately
produces three loose pieces that drift apart.

Three mid-typing states (valid-so-far / definite-error /
incomplete-but-plausible) drive four layered channels:
token-class colour and parse-error overlay (silent, always
on), hint panel ambient and Tab-triggered completion mode
(verbose, in the existing hint panel — no floating popups).

Schema-aware from day one via an IdentSlot taxonomy in the
parser (NewName / TableName / ColumnIn(TableRef::Earlier(N))
/ RelationshipName); every existing ident() call gets
audited and tagged. Completion candidates come from chumsky's
expected-token-set for keyword slots and from a new worker
request (ListNamesFor) for identifier slots.

Implementation lands in 8 green-after-each commits: theme
colours; input panel highlighting; echo line highlighting;
render-time parse + error overlay; hint panel ambient;
identifier-slot taxonomy + parser audit; schema query
plumbing; completion mode + key bindings. Estimated
1500-2500 lines across the eight stages.

Out of scope (deliberately): inline ghost text (could return
as a "most-likely" affordance later — fish-shell style),
fuzzy matching, punctuation completion, user-customisable
keybindings, SQL highlighting in advanced mode (waits on Q4).

docs/adr/README.md

@@ -27,3 +27,4 @@ This directory contains the project's ADRs, recorded per
 - [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-0022 — Ambient typing assistance: colour, hint panel, completion (I3 + I4)](0022-ambient-typing-assistance.md)