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.

src/ui.rs

@@ -1117,6 +1117,7 @@ mod tests {
                     primary_key: true,
                     unique: false,
                     default: None,
+                    check: None,
                 },
                 ColumnDescription {
                     name: "Name".to_string(),
@@ -1126,6 +1127,7 @@ mod tests {
                     primary_key: false,
                     unique: false,
                     default: None,
+                    check: None,
                 },
             ],
             outbound_relationships: Vec::new(),