Networking control protocol

[GregoryLi360]

Control and data pipe protocol between Arca and the Linux monitor.

Control protocol (spec in PROTOCOL.md):

• Wire format: 7-byte header (message_type, payload_len, request_id) plus fixed-layout payload.
• Message types: ListenRequest/Ok/Err, ConnectRequest/Ok/Err, AcceptRequest, IncomingConnection/Err, AcceptErr.
• Arca side: ArcaSession::bind / connect / accept mirror std::net.
• Linux side: Monitor::pump_once / serve_one / poll_accepts drive the kernel sockets.
• One static control pipe; per-connection data pipes allocated on demand.
• 'DataPipeInfo.pipe_id` is u64 to hold the SHM allocator offset (used by both sides to reconstruct their local pointer via from_offset).

Pipe and data-pipe (update) (spec in DATAPIPE.md):

• RingHeader adds writer_closed / reader_closed atomic flags in shared memory
• BidirectionalPipe exposes close_write/read(), is_peer_write/read_closed(), is_closed().
• SyncStream / AsyncStream rewritten to raw byte stream (dataframe framing removed), send / recv use shared-memory close flags for EOF and error detection.

See PROTOCOL.md and DATAPIPE.md for the full specs.