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/data.rs

@@ -375,7 +375,10 @@ fn require_ident(path: &MatchedPath, role: &'static str) -> Result<String, Valid
 /// Convert a `MatchedItem` whose kind is one of the `value_literal`
 /// variants (Word("null"|"true"|"false"), NumberLit, StringLit) to
 /// a `Value`. Returns None for non-value items.
-fn item_to_value(item: &MatchedItem) -> Option<Value> {
+///
+/// `pub(crate)` so `grammar::ddl` can reuse it when collecting a
+/// `default <literal>` column constraint (ADR-0029).
+pub(crate) fn item_to_value(item: &MatchedItem) -> Option<Value> {
     match &item.kind {
         MatchedKind::Word("null") => Some(Value::Null),
         MatchedKind::Word("true") => Some(Value::Bool(true)),