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.

tests/walking_skeleton.rs

@@ -252,6 +252,7 @@ fn fake_table(name: &str, columns: &[(&str, Type, bool)]) -> TableDescription {
                 primary_key: *pk,
                 unique: false,
                 default: None,
+                check: None,
             })
             .collect(),
         outbound_relationships: Vec::new(),
@@ -419,6 +420,7 @@ fn add_relationship_flow_shows_parent_side_with_inbound_section() {
             primary_key: true,
             unique: false,
             default: None,
+            check: None,
         }],
         outbound_relationships: Vec::new(),
         inbound_relationships: vec![RelationshipEnd {
@@ -470,6 +472,7 @@ fn add_relationship_flow_shows_inbound_section_on_parent() {
             primary_key: true,
             unique: false,
             default: None,
+            check: None,
         }],
         outbound_relationships: Vec::new(),
         inbound_relationships: vec![RelationshipEnd {