Improve task cancellation flow & lifecycle management

[ThangKM]

Improve task cancellation flow & lifecycle management

Summary

This PR enhances the cancellation and lifecycle management across CancelBag, StreamProducer, and ScreenActionStore.

Changes

CancelBag

• Auto-cleanup of completed tasks — A new watch() method awaits each task's result and removes it from storage upon completion, preventing unbounded growth of the cancellers dictionary.
• AnyHashable identifiers — Task identifiers are now AnyHashable instead of String, allowing actions (or any Hashable type) to be used directly as cancellation keys.
• Optional bag parameter — store(in:) now accepts an optional CancelBag?, making it more ergonomic when a cancel bag may not be present.
• isEmpty / count properties — Expose the current number of tracked tasks for inspection and testing.
• Added documentation comments throughout.

StreamProducer

• Extract StreamStorage — Moved continuation storage into a dedicated final class StreamStorage with a deinit that automatically finishes all streams, ensuring cleanup when the producer is deallocated.
• Deprecate nonIsolatedFinish() — Marked as deprecated since streams now auto-finish on deallocation.

ScreenActionStore

• Simplified protocol — Removed associatedtype AScreenState and binding(state:). The protocol now only requires receive(action:).
• nonisolatedReceive(action:canceller:) — New default extension method that dispatches an action from a nonisolated context, optionally storing the task in a CancelBag using the action itself as the cancellation identifier.

Tests

• Added testWatchTaskCompletedRemoveCancellerFromStorage verifying that completed tasks are automatically removed from the bag.

Commit Message

refactor: improve CancelBag lifecycle, extract StreamStorage, and simplify ScreenActionStore

- CancelBag: auto-remove completed tasks via watch(), add isEmpty/count,
  accept AnyHashable identifiers, make bag parameter optional
- StreamProducer: extract StreamStorage class with deinit cleanup,
  deprecate nonIsolatedFinish()
- ScreenActionStore: replace binding(state:) with receive(action:),
  add nonisolatedReceive helper with CancelBag support
- Add CancelBagTests for auto-removal of completed tasks