Refactor AsyncPipeWriterOutput for stream compatibility
- Reduce test chunk size to 256 bytes and update test names/comments
- Add sender-side diagnostic logging and unify with receiver logs
- Detect StreamPipeWriter at runtime and enforce sequential flush/acquire for streams
- Retain parallelism for pipe-based writers (Kestrel/SignalR)
- Add DEBUG-only diagnostics at key chunking points
- Minor code style cleanups and doc clarifications
- Add Bash command to fetch StreamPipeWriter.cs for reference