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, AcceptErr.
• Arca side: ArcaSession::bind / connect / accept mirror std::net.
• Linux side: Monitor::pump_once / serve_one / poll_accepts / event_loop drive the kernel sockets.
• One static control pipe; per-connection data pipes allocated on demand from BuddyAllocator.
• DataPipeInfo.shm_offset (u64) carries the BuddyAllocator byte offset; both sides call from_offset(shm_offset) to reconstruct their local pointer to the same physical memory.
• Monitor allocates each pipe's SHM with Box::new_zeroed_slice_in(BuddyAllocator); freed automatically when the pipe fully closes.
• event_loop drives control-frame dispatch and tcp↔pipe byte relay in one loop; half-close propagates in both directions between TCP and the shared-memory rings.

Pipe and data-pipe (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().
• Write trait adds write_all() default (spins on WouldBlock until all bytes are written).
• SyncStream / AsyncStream rewritten to raw byte stream; send / recv use the close flags for EOF and error detection; dataframe framing removed.
• SyncStream / AsyncStream carry shm_offset (u64) — the BuddyAllocator offset of the backing region.

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