constraints: add constraint / drop constraint on existing columns (ADR-0029 §2.2)
Adds the two commands for modifying a column's constraints after creation, completing ADR-0029's §2.2 surface. Grammar (dsl/grammar/ddl.rs): `add constraint <constraint> to <T>.<col>` reuses the §2.1 COLUMN_CONSTRAINT choice; `drop constraint <kind> from <T>.<col>` names only the kind. Both join the `add` / `drop` choices, discriminated by the `constraint` form word. AST (dsl/command.rs): `Command::AddConstraint` / `DropConstraint` plus the `Constraint` / `ConstraintKind` enums. Worker (db.rs): `do_add_constraint` / `do_drop_constraint` apply the change through the rebuild-table primitive. `add` runs the §5 dry-run first — `not null` / `unique` / `check` against a populated column are refused, before any write, with a pretty-printed table of offending rows. §9 redundant-on-PK declarations and §6 `default` on an auto-generated column are friendly refusals; dropping a constraint the column does not carry is likewise refused. Also fixes schema_to_ddl, which suppressed UNIQUE for every PK column — a compound-PK member is not individually unique, so an explicit UNIQUE on it must survive the rebuild. 23 tests added (6 grammar, 17 worker); 3 completion-test and 3 matrix snapshots updated for the new `constraint` subcommand.
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