grammar+db: 3i — insert_arity_mismatch diagnostic (ADR-0033 §8.1)

New dml_insert_arity_diagnostics pass (ERROR): when an explicit
(column_name_list) arity disagrees with a row's arity. VALUES tuples
are checked per-row (each offending tuple emits its own diagnostic on
its span; matched rows stay silent). INSERT … SELECT compares the
first SELECT leg's projection arity, anchored on the first projection
item; a WITH-prefixed row source is skipped (engine still reports it —
a false positive would be worse). No-column-list form deferred
(needs schema; outside the 3i gate).

The VALUES walk stops at the first depth-0 keyword so an ON CONFLICT
(col) conflict target / RETURNING tail is not mis-counted as a value
tuple (caught by the existing upsert_excluded tests during dev).

Catalog key diagnostic.insert_arity_mismatch (engine-neutral).
Tests (+7): single-row + matched + per-row multi-row; INSERT…SELECT
mismatch + matched; ON CONFLICT interaction (only the real tuple
flagged, clean case silent). 1587 pass / 0 fail / 1 ignored. Clippy
clean. Remaining 3i: not_null_missing (needs TableColumn
not_null+default), cross-cut verification, #12 UPSERT DO UPDATE
validation.

src/friendly/keys.rs

@@ -44,6 +44,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("diagnostic.cte_arity_mismatch", &["cte", "declared", "actual"]),
     ("diagnostic.duplicate_cte", &["name"]),
     ("diagnostic.eq_null", &[]),
+    ("diagnostic.insert_arity_mismatch", &["expected", "actual"]),
     ("diagnostic.like_numeric", &["column", "type"]),
     ("diagnostic.projection_alias_misplaced", &["alias", "clause"]),
     ("diagnostic.type_mismatch", &["column", "type"]),