AppStateTests: Tests AppState as isolated instances without touching
RenderNotifier.current. Removed .serialized (no shared state).
StateStorageIdentityTests: Replaced testEnvironment() with testStorage()
that only creates a StateStorage. No longer sets RenderNotifier.current.
StateBox.didSet harmlessly calls setNeedsRender() on the default global.
StatePropertyTests: The one test that needs RenderNotifier.current
(stateTriggerRender) now swaps it atomically — set, mutate, check,
restore in a single synchronous sequence with no yield points.
Root cause: All three suites ran .serialized internally but parallel
to each other. Since RenderNotifier.current is a process-global static
variable, concurrent writes caused one suite to overwrite another's
AppState, making needsRender checks fail intermittently.
phranck
5975820ed4