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.

src/app.rs

@@ -1421,6 +1421,16 @@ impl App {
                 RelationshipSelector::Named { .. } => (Operation::DropRelationship, None, None),
             },
             C::AddIndex { table, .. } => (Operation::AddIndex, Some(table.as_str()), None),
+            C::AddConstraint { table, column, .. } => (
+                Operation::AddConstraint,
+                Some(table.as_str()),
+                Some(column.as_str()),
+            ),
+            C::DropConstraint { table, column, .. } => (
+                Operation::DropConstraint,
+                Some(table.as_str()),
+                Some(column.as_str()),
+            ),
             C::DropIndex { selector } => match selector {
                 IndexSelector::Columns { table, .. } => {
                     (Operation::DropIndex, Some(table.as_str()), None)