constraints: CHECK — check (<expr>) at create table & add column (ADR-0029)

The fourth constraint. `check ( <expr> )` reuses the ADR-0026
WHERE-expression grammar via `Subgrammar`, so a check is
written in the same language as a `where` filter.

- Grammar: a `CHECK_CONSTRAINT` arm joins the shared
  constraint-suffix Choice; `consume_check_expr` extracts the
  parenthesised expression (paren-depth aware) into
  `ColumnSpec.check` / `Command::AddColumn.check`.
- Storage: the parsed `Expr` is compiled once to inline SQL
  (`compile_check_sql` — `compile_expr` + ADR-0028's
  param-inliner) and stored in that form everywhere — a new
  `check_expr` column in `__rdbms_playground_columns`,
  `project.yaml`'s `ColumnSchema.check`, and the column DDL
  emitted by `do_create_table` / `schema_to_ddl`.
- `add column … check` routes through the rebuild primitive
  (SQLite's `ALTER … ADD COLUMN` cannot carry it); a CHECK on
  a serial/shortid column is create-table-only and refused at
  add-column with a friendly message.
- `describe` surfaces the CHECK. ADR-0029 §7/§8 updated to the
  SQL-form decision — double-quoted identifiers, consistent
  with ADR-0028's `explain` display SQL.

1201 tests pass (+8); clippy clean.

docs/simple-mode-limitations.md

@@ -48,3 +48,8 @@ entry names the ADR that drew the boundary.
   added afterward with `add column`. Creating a table with a
   mix of PK and non-PK columns in one statement needs
   advanced-mode `CREATE TABLE` syntax.
+- **`check (<expr>)` constraints reuse the WHERE-expression
+  grammar** (ADR-0026), so the same limits apply: no scalar
+  functions (`length(x)`), no arithmetic. A check is a boolean
+  combination of column-vs-literal comparisons, `LIKE`, `IN`,
+  `BETWEEN`, and `IS NULL`.