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.

CLAUDE.md

@@ -178,15 +178,6 @@ not yet implemented:
   1–4 of ADR-0015). Pending pieces: `export` / `import` (Iter
   5), `--resume` + persistent input history hydration +
   migration framework scaffold (Iter 6).
-- **Complex WHERE expressions** (C5a): implemented through
-  ADR-0026 steps 1–4 — the stratified expression grammar
-  (AND/OR/NOT, the six comparisons, LIKE/IN/BETWEEN/IS NULL,
-  parentheses) reached through a new `Subgrammar` node, the
-  recursive `Expr` AST + parameterised SQL, and `where` /
-  `limit` on `show data`. Type-mismatched comparisons run
-  permissively (§7). Still pending: the §7 advisory
-  *flagging* of type mismatches / `= NULL`, the seam with
-  ADR-0027's diagnostics-severity model.
 - **SQL handling in advanced mode** (Q1): `sqlparser-rs` parser
   + a defined SQL subset (Q4 — its own ADR).
 - **Column drops/renames/type changes** (B2 / C2 partial): the