v0.7.1 — buffered writer (fewer syscalls)

Performance

• Buffered connection writer. The transport writer is wrapped in a reused
  16 KiB bufio.Writer, coalescing a DATA frame's 9-byte header and payload
  into 1 write syscall instead of 2. A target-load CPU profile showed the
  client is syscall-bound (~50% CPU in the socket write); a 7-DATA-frame body
  drops from ~16 write syscalls to 8, with no per-frame allocation. Flushed
  under wmu before every lock release and every flow-control wait — guarded by
  TestWriteBuffer_MultiChunkUpload_NoDeadlock.

No API changes. Full notes in CHANGELOG.md.