Matrix: insert Form C + update + delete coverage

34 new tests covering:
- Form C bare-value-list (happy path + Form-A-recovery + type-unaware grammar limitation per handoff §2.2)
- update with WHERE (column-narrowing invariant per handoff §1 bug E1; typed-slot prose for assignments and where filters)
- update --all-rows (filter-clause requirement per ADR-0014)
- delete with WHERE (column-narrowing; typed-slot prose for where filters)
- delete --all-rows

859 baseline -> 931 passing. No bugs surfaced — the data-mutation
command family was already well-shaped post-Phase-D.

tests/typing_surface/insert_form_c.rs

@@ -1 +1,109 @@
-//! Submodule stub — populated in subsequent tasks.
+//! Matrix coverage for `insert into T (vals)` (Form C — bare
+//! value list, no `values` keyword).
+//!
+//! Form C shares the `( ... )` opener with Form A but resolves
+//! the paren contents as values rather than column names. Per
+//! handoff-12 §2.2 the Form C path is *type-unaware* — its
+//! grammar uses the schemaless `INSERT_PAREN_LIST` shape, not
+//! the typed `column_value_list`. Type validation happens at
+//! bind time, not parse time.
+//!
+//! The previous commit's Form C/A disambiguation means
+//! column-shaped items (idents) inside the parens now flag as
+//! "did you mean Form A?". This file pins both the happy-path
+//! (literals only) and the Form-A-recovery (column-shaped
+//! items).
+
+use crate::typing_surface::*;
+use rdbms_playground::input_render::InputState;
+
+#[test]
+fn form_c_with_text_literals_parses() {
+    let schema = schema_text_pk();
+    let a = assess_at_end(
+        "insert into Items ('SKU-1', 'Widget')",
+        &schema,
+    );
+    assert!(matches!(a.state, InputState::Valid));
+    assert_eq!(a.parse_result.as_deref(), Ok("Insert"));
+    crate::snap!("form_c_text_literals", a);
+}
+
+#[test]
+fn form_c_with_mixed_literals_parses() {
+    let schema = schema_serial_pk();
+    let a = assess_at_end(
+        "insert into Customers (1, 'Alice', 'a@b.c')",
+        &schema,
+    );
+    assert!(matches!(a.state, InputState::Valid));
+    assert_eq!(a.parse_result.as_deref(), Ok("Insert"));
+    crate::snap!("form_c_mixed_literals", a);
+}
+
+#[test]
+fn form_c_with_null_first_parses() {
+    let schema = schema_serial_pk();
+    let a = assess_at_end(
+        "insert into Customers (null, 'Alice', 'a@b.c')",
+        &schema,
+    );
+    assert!(matches!(a.state, InputState::Valid));
+    crate::snap!("form_c_null_first", a);
+}
+
+#[test]
+fn form_c_with_column_shaped_item_flags_as_form_a_in_progress() {
+    let schema = schema_serial_pk();
+    let a = assess_at_end("insert into Customers (Name)", &schema);
+    assert!(
+        matches!(a.state, InputState::IncompleteAtEof),
+        "expected IncompleteAtEof (Form A recovery), got {:?}",
+        a.state,
+    );
+    assert_candidate_present(&a, &["values"]);
+    crate::snap!("form_c_column_shaped_recovery", a);
+}
+
+#[test]
+fn form_c_with_two_columns_flags_as_form_a_in_progress() {
+    let schema = schema_serial_pk();
+    let a = assess_at_end("insert into Customers (Name, Email)", &schema);
+    assert!(matches!(a.state, InputState::IncompleteAtEof));
+    assert_candidate_present(&a, &["values"]);
+    crate::snap!("form_c_two_columns_recovery", a);
+}
+
+#[test]
+fn form_c_type_unaware_grammar_accepts_decimal_for_int_column() {
+    // Form C's grammar uses INSERT_PAREN_LIST (the pre-Phase-D
+    // schemaless choice), so type mismatches aren't caught at
+    // parse time. Bind time catches them. Handoff §2.2
+    // documents this as known.
+    let schema = schema_serial_pk();
+    let a = assess_at_end(
+        "insert into Customers (3.14, 'Alice', 'a@b.c')",
+        &schema,
+    );
+    assert!(matches!(a.state, InputState::Valid));
+    crate::snap!("form_c_type_unaware", a);
+}
+
+#[test]
+fn form_c_in_progress_after_comma_is_incomplete() {
+    let schema = schema_serial_pk();
+    let a = assess_at_end("insert into Customers (1, ", &schema);
+    assert!(matches!(a.state, InputState::IncompleteAtEof));
+    crate::snap!("form_c_in_progress_after_comma", a);
+}
+
+#[test]
+fn form_c_in_progress_without_close_paren_is_incomplete() {
+    let schema = schema_serial_pk();
+    let a = assess_at_end(
+        "insert into Customers (1, 'Alice'",
+        &schema,
+    );
+    assert!(matches!(a.state, InputState::IncompleteAtEof));
+    crate::snap!("form_c_in_progress_no_close", a);
+}