Add facade.test.ts asserting the two invariants that keep lib/api.ts safe:
no method name is defined in two slices (a silent last-wins spread override),
and the `api` object is exactly the union of the slices (nothing added inline
in the barrel, nothing lost). Verified to have teeth — an injected duplicate
key fails the collision test with a named message.
Closes R7.5: the api layer is now per-domain modules behind a mechanically
guarded spread-only façade.