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/snapshots/typing_surface_matrix__typing_surface__insert_form_b__form_b_with_extra_value_for_serial_column_is_invalid@form_b_extra_serial_value.snap

@@ -1,37 +1,20 @@
 ---
 source: tests/typing_surface/insert_form_b.rs
-assertion_line: 186
+assertion_line: 193
 description: "input=\"insert into Customers values (1, 'Alice', 'a@b.c')\" cursor=50"
 expression: "& a"
 ---
 Assessment {
     input: "insert into Customers values (1, 'Alice', 'a@b.c')",
     cursor: 50,
-    state: DefiniteErrorAt(
-        30,
-    ),
+    state: Valid,
     hint: Some(
         Prose(
-            "for `Name`: Type a quoted string (e.g. 'Alice') or null (`id` auto-generated — skipped here; list columns explicitly, e.g. `insert into T (...) values (...)`, to set it.) (trying to write SQL? switch with `mode advanced`, or prefix `:` to run once)",
+            "without a column list, supply 2 value(s) for `Name`, `Email` — `id` auto-generated; 3 given (trying to write SQL? switch with `mode advanced`, or prefix `:` to run once)",
         ),
     ),
-    completion: Some(
-        Completion {
-            replaced_range: (
-                50,
-                50,
-            ),
-            partial_prefix: "",
-            candidates: [
-                Candidate {
-                    text: "null",
-                    kind: Keyword,
-                    mode: Both,
-                },
-            ],
-        },
-    ),
-    parse_result: Err(
-        "Invalid(definite)",
+    completion: None,
+    parse_result: Ok(
+        "Insert",
     ),
 }

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__insert_form_b__form_b_with_too_few_values_is_invalid_at_close_paren@form_b_too_few_values.snap

@@ -1,22 +1,20 @@
 ---
 source: tests/typing_surface/insert_form_b.rs
-assertion_line: 168
+assertion_line: 173
 description: "input=\"insert into Customers values ('Alice')\" cursor=38"
 expression: "& a"
 ---
 Assessment {
     input: "insert into Customers values ('Alice')",
     cursor: 38,
-    state: DefiniteErrorAt(
-        37,
-    ),
+    state: Valid,
     hint: Some(
         Prose(
-            "after `insert into Customers values ('Alice'`, expected `,` — usage: insert into <Table> [(<col>[, ...])] [values] (<value>[, ...])",
+            "without a column list, supply 2 value(s) for `Name`, `Email` — `id` auto-generated; 1 given",
         ),
     ),
     completion: None,
-    parse_result: Err(
-        "Invalid(definite)",
+    parse_result: Ok(
+        "Insert",
     ),
 }

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__insert_form_c__form_c_wrong_value_count_is_invalid@form_c_wrong_count.snap

@@ -1,22 +1,35 @@
 ---
 source: tests/typing_surface/insert_form_c.rs
-assertion_line: 101
+assertion_line: 104
 description: "input=\"insert into Customers ('Alice', 'a@b.c', 'extra')\" cursor=49"
 expression: "& a"
 ---
 Assessment {
     input: "insert into Customers ('Alice', 'a@b.c', 'extra')",
     cursor: 49,
-    state: DefiniteErrorAt(
-        39,
-    ),
+    state: Valid,
     hint: Some(
         Prose(
-            "after `insert into Customers ('Alice', 'a@b.c'`, expected `)` — usage: insert into <Table> [(<col>[, ...])] [values] (<value>[, ...])",
+            "without a column list, supply 2 value(s) for `Name`, `Email` — `id` auto-generated; 3 given",
         ),
     ),
-    completion: None,
-    parse_result: Err(
-        "Invalid(definite)",
+    completion: Some(
+        Completion {
+            replaced_range: (
+                49,
+                49,
+            ),
+            partial_prefix: "",
+            candidates: [
+                Candidate {
+                    text: "values",
+                    kind: Keyword,
+                    mode: Both,
+                },
+            ],
+        },
+    ),
+    parse_result: Ok(
+        "Insert",
     ),
 }