docs: ADR-0035 4a plan + 4a.2 split for composite UNIQUE / table CHECK

Add the sub-phase 4a implementation plan (docs/plans/), test-first,
mirroring the ADR-0033 DML sub-phase model: SqlCreateTable as its own
command executed structurally through the existing do_create_table
helper; shared-entry-word dispatch (SQL-first, simple fallback); the
type-alias resolver; IF NOT EXISTS no-op-with-note (CreateOutcome
enum); INTEGER PRIMARY KEY -> plain int; one-undo-step wiring.

Records the user-confirmed 4a/4a.2 split: composite UNIQUE(a,b) and
multi-column table CHECK move to a dedicated slice because they are the
first structures TableSchema cannot already represent, so they need a
persistence-model + round-trip extension rather than parse+execute
reuse. ADR-0035 §13 gains 4a.2; README sub-phase line updated in
lockstep.

docs/adr/0035-advanced-mode-sql-ddl.md

@@ -307,7 +307,18 @@ ADR-0033's structure:
 - **4a — Dispatch + `CREATE TABLE` core.** Advanced `create`
   dispatch; `SqlCreateTable` for columns + types (the §3 map) +
   column constraints + single/compound `PRIMARY KEY`, plus
-  `IF NOT EXISTS` (no-op-with-note, §4). No FK yet.
+  `IF NOT EXISTS` (no-op-with-note, §4). Single-column table-level
+  `UNIQUE`/`CHECK` normalise into the column; **no FK** (4b). Reuses
+  `do_create_table`.
+- **4a.2 — Composite `UNIQUE(a,b)` / multi-column table `CHECK`.**
+  Split out (2026-05-24, user-confirmed) because these are the first
+  structures the data model cannot already represent: `TableSchema`
+  has no slot for them. A self-contained slice that extends
+  `TableSchema` + the YAML round-trip + `read_schema` detection +
+  `do_create_table` DDL emission, with save/load/rebuild tests. (The
+  general rule: a DDL feature needs data-model work only when it
+  introduces a structure simple mode could never produce — cf. the
+  `UNIQUE`-index flag in 4d and the new rename op in 4h.)
 - **4b — Foreign keys in `CREATE TABLE`.** Inline `REFERENCES` +
   table-level `FOREIGN KEY` → relationship metadata, one undo step.
 - **4c — `DROP TABLE [IF EXISTS]`** → `SqlDropTable` (cascade parity;