feat: ADR-0035 4g — ALTER TABLE add/drop constraint + add FK

ALTER TABLE <T> ADD [CONSTRAINT <name>] (CHECK | UNIQUE | FOREIGN KEY)
and DROP CONSTRAINT <name>. ADD = table-CHECK + composite UNIQUE + FK
(ADD PRIMARY KEY and a named UNIQUE refused — composite UNIQUE is
anonymous in our model). Each ADD reuses a low-level path with a dry-run
guard (table-CHECK/UNIQUE rebuild; FK -> add_relationship, bare
REFERENCES -> parent single PK). DROP CONSTRAINT resolves the name to a
named table-CHECK then a child-side FK, else refuses. One undo step each.

Named table-CHECKs round-trip: a nullable `name` column on
__rdbms_playground_table_checks (rebuild-only arrival; a named add on a
pre-4g project is refused with a "rebuild first" hint) plus a project.yaml
check_constraints {expr, name} extension (bare-string form still reads).
The internal-__rdbms_* guard was folded into do_add_constraint /
do_add_relationship, completing that guard class.

Grammar: the action Choice keeps one branch per verb (add/drop/rename/
alter) with an inner Choice fanning out on the distinct second keyword,
since the walker's Choice does not backtrack between same-led branches.

Tests: 7 Tier-1 parse + 2 yaml round-trip + 1 internal-guard + 9 Tier-3
e2e. Help/usage refreshed; ADR-0035 §13 4g + README + requirements.md in
lockstep.

src/app.rs

@@ -1602,6 +1602,12 @@ impl App {
                     Some(table.as_str()),
                     Some(column.as_str()),
                 ),
+                AlterTableAction::AddTableConstraint { .. } => {
+                    (Operation::AddConstraint, Some(table.as_str()), None)
+                }
+                AlterTableAction::DropConstraint { .. } => {
+                    (Operation::DropConstraint, Some(table.as_str()), None)
+                }
             },
             C::SqlCreateTable { name, .. } => {
                 (Operation::CreateTable, Some(name.as_str()), None)