The `LOUDNESS_BACKFILL_WINDOW_AHEAD` constant, the predicate
`isTrackInsideLoudnessBackfillWindow`, and the id-list collector
`collectLoudnessBackfillWindowTrackIds` move into
`src/store/loudnessBackfillWindow.ts`.
`isTrackInsideLoudnessBackfillWindow` was rewritten to be pure — it
now takes the queue / queueIndex / currentTrack as explicit
parameters instead of reading `usePlayerStore.getState()` internally.
The one call site in `refreshLoudnessForTrack` reads state once and
passes it through. Net: zero behaviour change, clearer test surface
(no store mocks), no top-level coupling back to playerStore.
`prefetchLoudnessForEnqueuedTracks` stays in playerStore — it calls
`refreshLoudnessForTrack` which lives there too.
13 focused tests pin the window-slides-with-queueIndex contract, the
clamp at the end of the queue, the empty-input fallbacks, and the
duplicate collapse when currentTrack is also in the ahead range.
playerStore 3261 → 3238 LOC.
Frank Stellmacher