feat: ADR-0035 4a.3 — table-level / multi-column CHECK

Add table-level CHECK (e.g. `CREATE TABLE t (a int, b int, CHECK (a < b))`)
to advanced-mode SQL CREATE TABLE. Since SQLite exposes no PRAGMA for CHECK
constraints, a table-level CHECK cannot be read back from the engine and
becomes the source of truth in a new internal metadata table
`__rdbms_playground_table_checks (table_name, seq, check_expr)`.

- Grammar: new TABLE_CHECK element in ELEMENT_CHOICES.
- Builder: distinguishes a table-level CHECK from a column-level one by
  element position (no column-def open in the element), using depth-aware
  boundary tracking so a length-arg comma (`numeric(10,2)`) or a
  table-PRIMARY KEY's inner comma is not mistaken for an element separator.
- Worker: do_create_table emits the CHECK clauses and writes the metadata
  rows in its transaction; schema_to_ddl emits them identically on rebuild;
  read_schema / read_schema_snapshot read them from the metadata table;
  do_drop_table clears them.
- Persistence: TableSchema.check_constraints round-trips through project.yaml
  (#[serde(default)], optional on read), mirroring unique_constraints.
- Composite UNIQUE deliberately stays PRAGMA-detected (engine-reportable,
  unlike CHECK) — user-confirmed.

DA/runda round added cross-cutting tests and a forward-looking doc fix:
- table CHECK survives a rebuild triggered by `add column`, and a later
  rebuild_from_text (the ADR-0013 rebuild primitive uses a raw DROP, so the
  metadata rows keyed on the final name are preserved);
- dropping a column a table CHECK references fails cleanly (rollback, table
  intact); detection is 4e, friendly wording is H1;
- dropping a table clears its CHECK metadata (no orphan rows on re-create);
- amended ADR §6 so 4h's RENAME also updates the new metadata table.

20 Tier-3 + 9 grammar/builder + 2 YAML tests. Docs: ADR-0035 Status/§13/§6,
README index, requirements.md Q1. Help/usage skeleton + describe display of
table-level constraints deferred to 4i (symmetric with 4a.2).

Tests: 1769 passing, 0 failing, 1 ignored. Clippy clean.

docs/requirements.md

@@ -218,9 +218,11 @@ handoff-14 cleanup; 449 after B2/C2.)
   `CREATE TABLE` (ADR-0035, 2026-05-25 — executed structurally: columns
   + types + `NOT NULL`/`UNIQUE`/`PRIMARY KEY` + `IF NOT EXISTS` (4a),
   then per-column `DEFAULT`/`CHECK` (raw `sql_expr` text) and composite
-  `UNIQUE(a,b)` (4a.2)). Remaining DDL — table-level/multi-column
-  `CHECK` (4a.3), FK (4b), `DROP TABLE` (4c), indexes (4d),
-  `ALTER TABLE` (4e–4h) — is phased per ADR-0035 §13.)*
+  `UNIQUE(a,b)` (4a.2), then table-level/multi-column `CHECK` (4a.3 —
+  round-trips via the new `__rdbms_playground_table_checks` metadata
+  table, since the engine reports no CHECK constraints)). Remaining DDL
+  — FK (4b), `DROP TABLE` (4c), indexes (4d), `ALTER TABLE` (4e–4h) —
+  is phased per ADR-0035 §13.)*
 - [ ] **Q2** Non-standard syntax rejected with a clear message
   pointing at the supported subset.
   *(Design done — ADR-0030 §8: out-of-subset statements are