create table: column constraints — NOT NULL / UNIQUE / DEFAULT grammar (ADR-0029)

`create table … with pk` now parses the column-constraint
suffix; combined with the commit-1 db layer, a constrained
table works end to end.

- A shared constraint-suffix grammar fragment — `not null`,
  `unique`, `default <literal>` — sits after each column's
  `(type)` group; `build_create_table` walks the matched path
  per column and folds the constraints into `ColumnSpec`.
- §9 redundancy check: every `with pk` column is a primary-key
  column, so `not null` (any) and `unique` (single-column PK)
  are rejected with a friendly error
  (`parse.custom.constraint_redundant_on_pk`).
- `project.yaml` round-trip: `ColumnSchema` gains `not_null` /
  `default`; the YAML reader/writer and `build_read_schema`
  carry them, so `rebuild` / `export` / `import` preserve
  constraints.
- ADR-0029 §2.1's example corrected — `create table` columns
  are all PK columns, so its suffix is for `default` / `check`;
  `docs/simple-mode-limitations.md` records that non-PK
  columns at create time need advanced mode.

CHECK is deferred to the next commit. 1184 tests pass (+7);
clippy clean.

src/dsl/grammar/shared.rs

@@ -354,7 +354,10 @@ const FALLBACK_VALUE_LITERAL_INNER: Node = Node::Choice(FALLBACK_VALUE_LITERAL_C
 /// surface the generic "Type a value: number, 'text', …" prose
 /// here rather than the misleading `null`/`true`/`false`
 /// candidate trio.
-const FALLBACK_VALUE_LITERAL: Node = Node::Hinted {
+/// The schemaless value-literal slot. `pub(crate)` so the
+/// `default <literal>` column constraint (ADR-0029) can reuse
+/// it from `grammar::ddl`.
+pub(crate) const FALLBACK_VALUE_LITERAL: Node = Node::Hinted {
     mode: HintMode::ProseOnly("hint.value_literal_slot"),
     inner: &FALLBACK_VALUE_LITERAL_INNER,
 };