docs: mark H1a done via the ADR-0042 systematic pass

requirements.md H1a → [x]: the per-command near-miss matrix (entry words,
missing clauses, committed multi-forms, both modes) plus the gap fixes
(G1 `1:n relationship`, G2 select projection gloss, G3 mode-aware usage
showing all valid forms, G4 `with` CTE template, CROSS JOIN ON teaching
message) close the systematic pass. The advanced-SQL items the survey
flagged (INSERT…SELECT count, RETURNING scope) were verified already
present. One low-priority residual is deferred by decision (submit-time
expression first-set at non-projection positions).

ADR-0042: record the pre-existing `SELECT *` arity caveat (INSERT…SELECT
with a star projection isn't expanded for pre-flight arity; engine
catches it at execution — adjacent to ADR-0019 §OOS-2).

docs/adr/0042-h1a-parse-error-pedagogy-grammar-tree.md

@@ -266,7 +266,13 @@ Verifying each against the running app **reversed two of three**:
 - **INSERT…SELECT column-count** is *already handled* — a count
   mismatch fires `verdict = Error` with "the column list names N
   column(s) but M value(s) are given" (walker test
-  `insert_select_arity_mismatch_fires`). Not a gap.
+  `insert_select_arity_mismatch_fires`). It is a structural
+  list-vs-list check, so it fires even without a schema. Not a gap.
+  *Caveat (pre-existing, not addressed here):* a `SELECT *`
+  projection is not expanded for arity, so `insert into T (one_col)
+  select * from Multi` is not pre-caught — the engine rejects it at
+  execution. Star-expansion for pre-flight arity would be a separate
+  enhancement (and brushes ADR-0019 §OOS-2 engine-error territory).
 - **RETURNING scope** is *already handled* — at a bare `returning`
   position completion offers the table's columns; `returning
   <unknown>` fires the `unknown_column` diagnostic. Not a gap.