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/0026-complex-where-expressions.md

@@ -462,19 +462,20 @@ choices, and where they deviate from the design sketch above:
   column's type, exactly as the pre-ADR `where col = val`
   slot did. The operand grammar carries no validators —
   permissive per §7.
-- **Step 5 deferred to ADR-0027.** The §7 *behaviour*
-  relaxation is done: `bind_where_literal` binds a
-  type-mismatched WHERE literal by its syntactic shape, and
-  the pre-ADR bind-time rejection is gone. The §7
-  *diagnostic flagging* — surfacing a type-mismatched
-  comparison or `= NULL` as an error-class highlight + hint
-  — is the seam with ADR-0027, which designs the walker
-  diagnostics-severity model that flagging belongs in (and
-  whose WARNING severity is defined to have "no triggers
-  until ADR-0026 is implemented"). Building the flagging as
-  a standalone mechanism first would be reworked by
-  ADR-0027; the recommendation is to implement it as the
-  first triggers of ADR-0027's model.
+- **Step 5 — done, as part of ADR-0027.** The §7
+  *behaviour* relaxation landed with steps 1–4:
+  `bind_where_literal` binds a type-mismatched WHERE literal
+  by its syntactic shape, and the pre-ADR bind-time
+  rejection is gone. The §7 *diagnostic flagging* — a
+  type-mismatched comparison or `= NULL` as a flagged
+  finding — was folded into ADR-0027's walker
+  diagnostics-severity model rather than built as a
+  standalone mechanism (ADR-0027's WARNING severity is
+  defined to have "no triggers until ADR-0026 is
+  implemented" — those triggers are exactly these). It
+  surfaces as a `Severity::Warning` `Diagnostic`, computed
+  post-walk from the built `Expr` against the table's
+  column types — see ADR-0027.
 
 ## See also