Replace hot-path bytes.NewBuffer usage with slice-backed readers and
writers, and build FlexBuffer-backed read/write contexts by value.
Remove defer from the targeted per-message and per-batch write paths,
return pooled buffers explicitly after downstream writes complete, and
keep TCP/UDP prelude behavior unchanged.
Add focused regression tests for stream, UDP, and batch masquerade and
prelude writes, and fix write-side bookkeeping to emit the actual
encoded bytes.
