DA pass on 3i. Fix: build_schema_cache set not_null = c.notnull ||
c.primary_key, which would false-flag an omitted `int` PK as a
not_null_missing WARNING — but an int PK is an INTEGER PRIMARY KEY
rowid alias that auto-fills (and SQLite's PK-NULL quirk means a PK
isn't implicitly NOT NULL anyway). Use c.notnull alone (ADR-0033
§8.3 "declared NOT NULL"): faithful and false-positive-free.
Arity-walk hardening (same class as the ON CONFLICT regression the
existing tests caught mid-3i): RETURNING after VALUES is a depth-0
keyword that ends the tuple list (only the real tuple is flagged),
and a comma nested in a function-call value (depth ≥ 2) does not
inflate the tuple's value count.
Tests (+2). 1598 pass / 0 fail / 1 ignored. Clippy clean.