Port Tauri's prefetch + gapless design into the Slint playback
controller. Gapless only works when the upcoming track is a cache hit
(plays via play_data), so prefetch is a prerequisite.
Prefetch: after_track_change now calls kick_prefetch, which peeks the
next 2 upcoming queue tracks and background-downloads each uncached,
non-local one into the player cache via Player::prefetch_into_cache.
Concurrency is bounded to 2 by a shared semaphore. Mirrors Tauri's
spawn_v2_prefetch (PREFETCH_LOOKAHEAD / MAX_CONCURRENT_PREFETCH).
Gapless trigger: the poll loop watches the player event; when
gapless_ready is set and gapless_next_track_id is 0, it resolves the
next upcoming track, fetches its bytes (L1 -> L2 -> CMAF) via
fetch_for_gapless, and calls Player::play_next. A gapless_requested_for
guard stops the 450ms ticker from re-firing while the download runs;
the current track is never queued as its own next.
Seamless-transition detection: when the engine performs the gapless
handoff the track changes while is_playing stays true (no stop). The
loop detects the track-id change and advances the core queue pointer,
refreshes now-playing metadata and the sidebar, and kicks prefetch for
the new track's successors -- without calling the audible play path.
The existing stop-then-play auto-advance still handles the non-gapless
(first/streamed track) case; gapless_requested_for is reset there too.
cc
vicrodh
4fb9270513