grammar: 3c — INSERT … SELECT row source (ADR-0033 §4)

Make the INSERT row source a Choice between the VALUES clause and
Subgrammar(&sql_select::SQL_SELECT_COMPOUND). SQL_SELECT_COMPOUND
is itself a Choice that admits a leading WITH, so a WITH-prefixed
SELECT row source (R4) parses through it for free; the two
branches start on disjoint keywords (values vs select/with) so the
Choice never ambiguously commits. No worker change — do_sql_insert
already executes the validated SQL and re-persists, and the engine
handles insert-from-query.

Tests: grammar accept (plain / column-list+projection / WITH-
prefixed / trailing-semi) and reject (__rdbms_* on the SELECT's
FROM slot, incomplete select); integration parse-path lowering +
worker round-trip (rows land, CSV re-persisted) incl. R4 WITH end-
to-end; walker cross-cut that the Phase-2 unknown_column diagnostic
fires on the INSERT…SELECT projection; DA-gate test that a self-
sourced INSERT…SELECT runs as a plain insert (no cascade summary —
that is DELETE-only). Still behind the dev `sqlinsert` entry word
(shared `insert` is 3j). 1493 tests green, clippy clean.

src/dsl/walker/mod.rs

@@ -3915,6 +3915,24 @@ mod tests {
         );
     }
 
+    #[test]
+    fn insert_select_unknown_projection_column_is_error() {
+        // ADR-0033 sub-phase 3c cross-cut: the Phase-2
+        // schema-existence pass fires on a SELECT row source
+        // embedded in an INSERT (no re-implementation needed).
+        // `nonexistent_col` is not a column of `a`.
+        let schema = two_table_schema();
+        let diags = diag_keys(
+            "sqlinsert into b select nonexistent_col from a",
+            &schema,
+        );
+        assert!(
+            diags.iter().any(|d| d.contains("no such column")),
+            "expected unknown_column on the INSERT…SELECT projection; \
+             got {diags:?}",
+        );
+    }
+
     #[test]
     fn cte_name_is_valid_table_source() {
         let schema = schema_with("base", &[("id", Type::Int)]);