ADR-0027: existing-cases sweep + docs (step F)

Sweep: input_verdict tests confirm the schema-existence check
fires across the identifier-taking commands — unknown table
on drop / show / add column, unknown column on drop column /
update — and that known references stay clean. The Step B
check is grammar-generic, so this is verification + coverage
rather than new code.

Docs: requirements.md S6 -> [x], baseline 1096; CLAUDE.md
deferred list reconciled (C5a and S6 are done — removed);
ADR-0026's as-built note updated (step 5 shipped via
ADR-0027); ADR-0027 gains an As-built notes section
recording the post-walk diagnostics realization, the
pre-rendered message, the timeout-based debounce, coarse
WARNING spans, and the deferred highlight/hint wiring.

docs/adr/0027-input-validity-indicator.md

@@ -229,6 +229,66 @@ ADR-0026's expression diagnostics (type mismatch, `= NULL`)
 land with that ADR's implementation and feed the WARNING
 severity.
 
+## As-built notes (2026-05-19)
+
+All five build-order steps are implemented, and — because
+ADR-0026 deferred its own step 5 — the ADR-0026 §7
+expression diagnostics (type mismatch, `= NULL`) were folded
+in here as the WARNING severity's first triggers. Realization
+choices and deviations from the sketch above:
+
+- **Diagnostics are computed post-walk, not emitted during
+  the walk.** §2 says "diagnostics are emitted there [in the
+  walker]". As built, the walk produces the `MatchedPath` and
+  the `Command` as before; `walk()` then runs two post-walk
+  passes — `schema_existence_diagnostics` over the matched
+  path, and `expr_warnings` over the built `Expr` — and
+  stores the result in `WalkResult::diagnostics`. This keeps
+  diagnostics off the walker's speculative-rollback paths
+  (a `Choice` branch that matches an ident then rolls back
+  would otherwise leak a diagnostic); the post-walk passes
+  see only the surviving terminals. Same single schema-aware
+  context, one logical pass later.
+- **`Diagnostic` carries a pre-rendered `message: String`,**
+  not the `{severity, span, message_key}` of §2. Each
+  diagnostic source catalog-translates at creation. This
+  sidesteps the awkwardness of a parse-style message having
+  no single static key, and the model stays "roughly
+  `{severity, span, message}`".
+- **The parse outcome is not a synthetic `Diagnostic`.** The
+  indicator reads it straight from `WalkOutcome` (§2's
+  "highest severity across the outcome *and* the
+  diagnostics"); `diagnostics` carries only the schema-aware
+  findings layered on a structurally-valid parse. Schema-
+  existence therefore runs only on a `Match`.
+- **`MatchedKind::Ident` gained an `IdentSource` field** so
+  the post-walk schema check can tell a table reference from
+  a column reference from an invented name without
+  hard-coding role strings.
+- **The debounce uses `tokio::time::timeout` per event-loop
+  iteration,** not `tokio::select!` — it keeps the existing
+  ~100-line event-handler body un-reindented. `recv` is
+  time-boxed only while a recompute is owed; an idle session
+  still blocks plainly. Any event resets the window (not
+  only keystrokes) — a slight over-reset, harmless in
+  practice.
+- **WARNING spans are coarse** — the whole WHERE clause. The
+  `Expr` AST carries no source spans, and the indicator
+  reads only severity. Precise per-literal spans (for a
+  future warning-highlight overlay) are a refinement.
+- **Highlight / hint integration beyond the indicator** —
+  §2 notes highlighting and the hint panel "read the
+  individual diagnostics". The indicator and the model
+  ship; routing diagnostic spans into the per-byte highlight
+  overlay, and diagnostic messages into the hint panel
+  beyond the existing parse-error path, is a follow-up (the
+  build order did not enumerate it as a step).
+- **Debounce timing is not unit-tested** — it is async
+  event-loop glue. `App::input_validity_verdict` (the pure
+  verdict) and the indicator rendering are covered;
+  `input_verdict` has a parse-outcome / schema-existence /
+  expression-warning / existing-cases-sweep test set.
+
 ## See also
 
 - ADR-0003 — input modes; the input field and its mode