Expand description
Length-prefixed SessionTransport over tokio::net::TcpStream.
SessionTransport is message-oriented (returns one frame per recv_bytes),
while TCP is a stream. This adapter inserts a 4-byte big-endian length prefix
before each frame so the trait contract is preserved.
Phase 2.1: the receive path keeps a single persistent BytesMut
accumulator across recv_bytes calls. Each frame is split_to-ed off
into an owned Bytes which the caller takes — zero-copy from the
accumulator to the returned frame, no per-packet Vec::new alloc.