Dropping the client's channel sender never closed the socket (the
server's reader thread keeps the connection open), so the client froze
waiting for frames. The server now sends a bye message; the client
exits cleanly on it and its socket close lets the server clean up.
Messages from already-disconnected clients are ignored. Verified
end-to-end with the real client in a PTY (exits 0, daemon survives).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>