Add three public Player methods that drive gapless playback without
touching the audio thread:
- prefetch_into_cache: download an upcoming track fully into L1/L2
cache so it later plays via play_data (cache hit) rather than the
streaming path. CMAF download_full first, legacy full download as
fallback. No-ops on streaming_only / already-cached / in-flight.
- is_track_cached: cheap L1/L2 membership check for the controller.
- fetch_for_gapless: resolve a track's bytes for a gapless handoff
via L1 memory then L2 disk then CMAF download_full. Ports the
L1/L2/CMAF tiers of Tauri's v2_play_next_gapless.
All three reuse existing primitives (audio_cache, download_audio);
the protected audio-thread command handlers are untouched.
cc
vicrodh
3a6aae19a3