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.

tests/typing_surface/mod.rs

@@ -208,6 +208,8 @@ fn command_kind_label(cmd: &rdbms_playground::dsl::Command) -> String {
         DropRelationship { .. } => "DropRelationship".into(),
         AddIndex { .. } => "AddIndex".into(),
         DropIndex { .. } => "DropIndex".into(),
+        AddConstraint { .. } => "AddConstraint".into(),
+        DropConstraint { .. } => "DropConstraint".into(),
         ShowTable { .. } => "ShowTable".into(),
         Insert { .. } => "Insert".into(),
         Update { .. } => "Update".into(),

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__add_relationship__after_add_offers_relationship_branch@after_add.snap

@@ -18,6 +18,10 @@ Assessment {
                     text: "index",
                     kind: Keyword,
                 },
+                Candidate {
+                    text: "constraint",
+                    kind: Keyword,
+                },
                 Candidate {
                     text: "1:n",
                     kind: Keyword,
@@ -42,6 +46,10 @@ Assessment {
                     text: "index",
                     kind: Keyword,
                 },
+                Candidate {
+                    text: "constraint",
+                    kind: Keyword,
+                },
                 Candidate {
                     text: "1:n",
                     kind: Keyword,

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__index_ops__after_add_offers_index_branch@after_add_index_branch.snap

@@ -18,6 +18,10 @@ Assessment {
                     text: "index",
                     kind: Keyword,
                 },
+                Candidate {
+                    text: "constraint",
+                    kind: Keyword,
+                },
                 Candidate {
                     text: "1:n",
                     kind: Keyword,
@@ -42,6 +46,10 @@ Assessment {
                     text: "index",
                     kind: Keyword,
                 },
+                Candidate {
+                    text: "constraint",
+                    kind: Keyword,
+                },
                 Candidate {
                     text: "1:n",
                     kind: Keyword,

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__index_ops__after_drop_offers_index_branch@drop_index_branch.snap

@@ -26,6 +26,10 @@ Assessment {
                     text: "index",
                     kind: Keyword,
                 },
+                Candidate {
+                    text: "constraint",
+                    kind: Keyword,
+                },
             ],
             selected: None,
         },
@@ -54,6 +58,10 @@ Assessment {
                     text: "index",
                     kind: Keyword,
                 },
+                Candidate {
+                    text: "constraint",
+                    kind: Keyword,
+                },
             ],
         },
     ),