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/friendly/keys.rs

@@ -207,12 +207,14 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     // code, not the catalog, because spacing is alignment-
     // sensitive in the multi-entry case.
     ("parse.usage.add_column", &[]),
+    ("parse.usage.add_constraint", &[]),
     ("parse.usage.add_index", &[]),
     ("parse.usage.add_relationship", &[]),
     ("parse.usage.change_column", &[]),
     ("parse.usage.create_table", &[]),
     ("parse.usage.delete", &[]),
     ("parse.usage.drop_column", &[]),
+    ("parse.usage.drop_constraint", &[]),
     ("parse.usage.drop_index", &[]),
     ("parse.usage.drop_relationship", &[]),
     ("parse.usage.drop_table", &[]),
@@ -385,6 +387,19 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
         &["count", "action", "child_table", "rel", "on_delete"],
     ),
     // ---- change-column dry-run diagnostics (per ADR-0017) ----
+    // ---- add-constraint dry-run diagnostics (per ADR-0029 §5) ----
+    (
+        "db.diagnostic.add_check_summary",
+        &["table", "column", "total", "rule"],
+    ),
+    (
+        "db.diagnostic.add_not_null_summary",
+        &["table", "column", "total"],
+    ),
+    (
+        "db.diagnostic.add_unique_summary",
+        &["table", "column", "total"],
+    ),
     ("db.diagnostic.force_conversion_hint", &[]),
     ("db.diagnostic.header_becomes", &[]),
     ("db.diagnostic.header_from", &[]),