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.

tests/typing_surface/insert_form_c.rs

@@ -85,18 +85,21 @@ fn form_c_rejects_number_for_text_column() {
 
 #[test]
 fn form_c_wrong_value_count_is_invalid() {
-    // Customers Form C expects exactly two values (id:serial
-    // skipped). Three values is a count mismatch — caught at
-    // parse time now.
+    // Customers Form C expects exactly two values (id:serial skipped).
+    // Three values is a count mismatch. As of issue #17 it parses
+    // structurally (so the friendly arity diagnostic fires); the
+    // user-facing invalidity is the validity verdict (`[ERR]`).
     let schema = schema_serial_pk();
-    let a = assess_at_end(
-        "insert into Customers ('Alice', 'a@b.c', 'extra')",
-        &schema,
-    );
-    assert!(
-        !matches!(a.state, InputState::Valid),
-        "Form C with too many values must be invalid, got {:?}",
-        a.state,
+    let input = "insert into Customers ('Alice', 'a@b.c', 'extra')";
+    let a = assess_at_end(input, &schema);
+    assert_eq!(
+        rdbms_playground::dsl::walker::input_verdict_in_mode(
+            input,
+            Some(&schema),
+            rdbms_playground::mode::Mode::Simple,
+        ),
+        Some(rdbms_playground::dsl::walker::Severity::Error),
+        "Form C with too many values must light the [ERR] verdict",
     );
     crate::snap!("form_c_wrong_count", a);
 }