signUpUseCase now takes an IEventBus and publishes userSignedUpEvent
after creating the user (synthesizing email from username since auth
is username-based). Use case mocks-default in module.ts get a fresh
InMemoryEventBus per resolution; bind-production / bind-dev-seed wire
the shared bus passed by bindAll. Tests updated to inject
RecordingEventBus, including a new test that asserts publish on
success and silence on failure.