feat(auth): signUp publishes userSignedUpEvent

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.

packages/auth/src/di/bind-dev-seed.ts

@@ -81,7 +81,7 @@ export async function bindDevSeedAuth(
     withCapture(
       logger,
       { feature: "auth", layer: "use-case", name: "auth.signUp" },
-      signUpUseCase(repo, authService),
+      signUpUseCase(repo, authService, bus),
     ),
   );
   const wrappedSignOut = withSpan(