refactor: Implement high-performance State Machine for flatMapLatest

- Replaces AsyncThrowingStream implementation with a custom AsyncSequence, Storage, and StateMachine.
- Implements explicit state management using Lock for thread safety.
- Handles concurrency between outer and inner sequences robustly.
- Ensures correct cancellation propagation and error handling.
- Verified with test_interleaving_race_condition.

Author: peterfriese