Adds weft-compositor/src/appd_ipc.rs: WeftAppdIpc state, setup() registers a calloop
UnixListener source. Accepted connections are registered as edge-triggered read sources.
Incoming AppdToCompositor frames are decoded and dispatched; AppSurfaceCreated records
pid->session mapping in pending_pids for later wl_surface association. Wires into both
the DRM and Winit backends. Socket path: /weft/compositor.sock or
WEFT_COMPOSITOR_SOCKET override.
