transport_server_create/start: binds TCP, spawns accept thread, closes
excess inbound connections when max_connections is reached.
transport_connect: outbound TCP, spawns read thread before returning.
transport_send_frame: packs 8-byte header with serial put_*, then writes
header + payload under a per-connection mutex (thread-safe).
Read thread: reads header, validates payload_length <= max_payload, mallocs
payload, calls on_frame (callback owns and must free payload). On error or
disconnect calls on_disconnect then frees conn.
transport_cli: server mode echoes received frames; client mode sends 3
test frames and prints echoes.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
