feat: support control command

[fengys1996]

Summary

Add a control message channel to the rad TCP protocol. LSP frames use RadMessage::Lsp as before; out-of-band control messages use RadMessage::Control. The entire pipeline is also lifted from raw Vec<u8> to typed LspFrame/RadMessage structs.

Key changes

• Rad wire format — New RadFrameCocdec (encode + decode). 5-byte header (1 byte kind + 4 bytes body len), JSON body. Supports Lsp and Control message kinds.
• rad status command — Client sends StatusRequest, server responds with each instance's workspace, pid, clients, idle time, and health, printed in a human-readable format.
• Instance status snapshot — InstanceManager::status() iterates all managed instances to collect runtime state.
• Typed pipeline — Channels upgraded from Vec<u8> to LspFrame (LSP layer) and RadMessage (protocol layer). RoutedPacket, ClientHandle, and related types use structured types, eliminating scattered serialization calls.
• LspSender wrapper — Replaces the to_client channel and forward_lsp_to_writer intermediate task. The instance layer only depends on send(LspFrame), completely unaware of RadMessage.
• Client framed I/O — Client-side stdin/server reads and writes migrated from raw read/write_all to FramedRead/FramedWrite.