Introduce a read/write lock abstraction and integrate it into `YamuxStream` so that simultaneous
reads and writes do not interleave, eliminating potential data corruption and race conditions.
Major changes:
- Abstract `ReadWriteLock` into its own util module
- Integrate locking into YamuxStream for `write` operations
- Ensure tests pass for lock correctness
- Fix lint & type issues discovered during review
Closes#793
