feat: bring simple-mode insert arity diagnostics to parity with advanced

A wrong-count simple-mode insert now shows the friendly per-column arity
message at typing time (instead of a bare "expected `,`/`)`") and is
blocked from dispatch at submit — unifying simple and advanced mode onto
the one ADR-0027 model (structural parse + ERROR diagnostic), where they
had diverged.

Grammar: a simple-mode-only arity gate (dsl_insert_value_list) routes a
wrong-count DSL insert tuple to the type-blind fallback so it matches
structurally and the per-tuple arity diagnostic fires. The gate is gated
to simple mode, so advanced behaviour is unchanged. count_tuple_values
and the target-column selection (insert_target_columns) are now shared
by both grammars.

Diagnostic: dml_insert_arity_diagnostics is mode-aware — advanced Form B
expects all columns; simple Form B/C expects the user-fillable columns
(serial/shortid auto-fill). It counts the DSL Form A role and scans the
keyword-less Form C tuple. New catalog keys name the fillable/auto split
and the all-auto-table case.

Submit: a wrong-count DSL insert now parses Ok + carries the ERROR
diagnostic, so a unified Ok-arm pre-flight (dsl_insert_count_mismatch_notes)
blocks dispatch and teaches; the previous Err-arm note retires.
advanced_alternative_note's gate now reads the validity verdict so it
still fires for the parse-Ok-with-error shape.

Docs: ADR-0036 Amendment 2 (+ README index) and requirements.md H1a.

docs/requirements.md

@@ -540,10 +540,15 @@ handoff-14 cleanup; 449 after B2/C2.)
   names the schema-correct next token (`,` between values, `)`
   after the last) instead of the type-blind close-paren, and so a
   wrong-arity closed tuple surfaces the real parse error rather
-  than a misleading "submit with Enter"). A systematic pass is
-  still pending; simple-mode wrong-count tuples still get a generic
-  expected-token message rather than the friendly arity diagnostic
-  advanced mode shows (issue #17).
+  than a misleading "submit with Enter"; **issue #17 / ADR-0036
+  Amendment 2, 2026-05-29** then brought the §8.1 arity diagnostic
+  to **simple mode** at parity with advanced — a wrong-count DSL
+  insert (Form A/B/C) now fires the friendly *"N value(s) for
+  `col`…"* message at typing time, counted against the user-fillable
+  columns, with `serial`/`shortid` auto-fill named; new keys
+  `diagnostic.insert_arity_mismatch_form_b_simple` /
+  `diagnostic.insert_arity_mismatch_all_auto`). A systematic pass is
+  still pending.
 - [ ] **H2** `hint` provides contextual help for the current
   input or the most recent error.
 - [ ] **H3** `help` provides general reference and per-command