feat: ADR-0035 4b — foreign keys in CREATE TABLE

Add foreign keys to advanced-mode SQL CREATE TABLE — the SQL spelling of
an ADR-0013 named relationship, created in the same transaction as the
table (one undo step).

- Grammar: inline `<col> … REFERENCES <parent>[(<col>)] [ON DELETE/UPDATE
  …]` (a new column constraint) and table-level `[CONSTRAINT <name>]
  FOREIGN KEY (<col>) REFERENCES …` (two new element branches — both
  start on a concrete keyword, never a leading Optional, which would
  abort the element Choice). Referential clauses reuse
  shared::REFERENTIAL_CLAUSES.
- Builder: greedy FK-clause consumption (parens consumed internally so
  they don't perturb the 4a.3 element-boundary depth tracker); inline FK
  auto-named, table FK takes an optional CONSTRAINT name.
- Worker: do_create_table resolves + validates each FK before building
  the DDL (self-ref validates against the in-statement columns/PK; bare
  REFERENCES resolves to the parent's single-column PK, composite ->
  error; PK-target + Type::fk_target_type compatibility), emits the
  FOREIGN KEY clause identically to schema_to_ddl, and writes the
  relationship metadata in the create transaction.
- Reuse: name/uniqueness/metadata-insert/type-compat factored into shared
  helpers; do_add_relationship refactored to use them.
- FKs round-trip via the existing relationship plumbing (no new
  persistence structures); describe surfaces the relationship.

Self-references and bare `REFERENCES <parent>` supported (user-confirmed).
Self-ref pre-submit indicator wrinkle deferred to 4i (tracked in ADR §13,
a code comment, and the plan).

DA/runda round added cross-cutting probes (FK survives the add-column
rebuild + a later rebuild_from_text; referential actions survive rebuild;
drop-child clears the relationship; drop-parent refused; bare self-ref
resolves to own PK) — all green, no fixes needed.

27 new tests (grammar/builder + Tier-3). Docs: ADR-0035 Status/§13,
README, requirements.md Q1.

Tests: 1795 passing, 0 failing, 1 ignored. Clippy clean.

docs/requirements.md

@@ -220,9 +220,12 @@ handoff-14 cleanup; 449 after B2/C2.)
   then per-column `DEFAULT`/`CHECK` (raw `sql_expr` text) and composite
   `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.)*
+  table, since the engine reports no CHECK constraints), then foreign
+  keys (4b — inline `REFERENCES` + table-level `FOREIGN KEY` → ADR-0013
+  named relationships in the create transaction; self-references and
+  bare `REFERENCES <parent>` supported)). Remaining DDL — `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