Add Squeezelite (Slim) PCM sink
Introduce a squeezelite sink: add rockbox-slim crate implementing a
broadcast buffer, Slim protocol server and HTTP audio endpoint; add
firmware pcm-squeezelite.c and register PCM_SINK_SQUEEZELITE. Add FFI
bindings, sys constant, and CLI force-link. Update settings to accept
audio_output = "squeezelite" and expose squeezelite_port /
squeezelite_http_port. Defaults: Slim 3483, HTTP 9999; streams raw S16LE
stereo to squeezelite clients.