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/friendly/keys.rs

@@ -227,6 +227,8 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("parse.caret", &["padding"]),
     // Custom (try_map / source-slice) error messages raised
     // by the DSL parser. See `parse.custom.*` in the catalog.
+    ("parse.custom.alter_add_primary_key", &[]),
+    ("parse.custom.alter_named_unique", &[]),
     ("parse.custom.bind_type_mismatch", &["found", "expected"]),
     ("parse.custom.change_column_flags_exclusive", &[]),
     ("parse.custom.constraint_redundant_on_pk", &["column", "constraint"]),