feat: ADR-0035 4a.2 — per-column CHECK/DEFAULT + composite UNIQUE

Advanced-mode SQL CREATE TABLE gains the constraints that need no new
internal table (the 4a.2 slice):

- Grammar (sql_create_table.rs): column-level DEFAULT/CHECK and
  table-level UNIQUE(cols). DEFAULT is a literal or a *parenthesised*
  expression (standard SQL) — a bare sql_expr greedily eats a following
  NOT (NOT IN/LIKE/BETWEEN), breaking `DEFAULT 0 NOT NULL`; the parens
  bound it. CHECK is paren-bounded already.
- Builder (ddl.rs): captures CHECK/DEFAULT raw SQL text by byte span
  (sql_expr builds no AST) via capture_parenthesised_span /
  capture_expr_span; routes single-column table UNIQUE into the
  column's flag and composite UNIQUE into unique_constraints.
- Command/worker: ColumnSpec gains check_sql/default_sql (raw, preferred
  over the typed Expr/Value); Command::SqlCreateTable + Request +
  do_create_table gain unique_constraints; do_create_table emits raw
  CHECK/DEFAULT and composite UNIQUE clauses.
- Round-trip (part D): ReadSchema/TableSchema gain unique_constraints;
  read_schema detects composite UNIQUE via PRAGMA index_list origin 'u'
  (single-column still folds to the column flag); schema_to_ddl emits
  them; YAML RawTable/write_table round-trips (optional-on-read).
  CHECK round-trips via __rdbms_playground_columns.check_expr, DEFAULT
  via PRAGMA table_info — no new metadata table.

Table-level/multi-column CHECK remains 4a.3 (rejected "not yet
supported"); FK is 4b.

Tests: +7 builder (raw-text capture incl. the DEFAULT 0 NOT NULL
boundary the fix was found by; single/composite UNIQUE routing) and +4
Tier-3 (CHECK enforced, DEFAULT applied, composite UNIQUE enforced, and
all three survive a rebuild — the part-D round-trip). 1752 pass / 0 fail
/ 1 ignored; clippy clean. Plan + requirements.md updated.

docs/requirements.md

@@ -215,11 +215,12 @@ handoff-14 cleanup; 449 after B2/C2.)
   the same completion / highlighting / hints as the DSL (ADR-0001's
   `sqlparser-rs` reservation is superseded). Implemented so far: full
   `SELECT` (ADR-0032), `INSERT` / `UPDATE` / `DELETE` (ADR-0033), and
-  `CREATE TABLE` (ADR-0035 sub-phase 4a, 2026-05-25 — columns + types +
-  `NOT NULL` / `UNIQUE` / `PRIMARY KEY` + `IF NOT EXISTS`, executed
-  structurally). Remaining DDL — `CREATE TABLE` constraints (4a.2),
-  FK (4b), `DROP TABLE` (4c), indexes (4d), `ALTER TABLE` (4e–4h) — is
-  phased per ADR-0035 §13.)*
+  `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.)*
 - [ ] **Q2** Non-standard syntax rejected with a clear message
   pointing at the supported subset.
   *(Design done — ADR-0030 §8: out-of-subset statements are