refactor(player): E.14 — extract loudness-backfill-window helpers (#577)

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.

src/store/loudnessBackfillWindow.test.ts

@@ -0,0 +1,91 @@
+/**
+ * Pure functions over a queue slice: the "is this id inside the prefetch
+ * window?" check and the "give me the window's id list" collector. Window
+ * = current track + next `LOUDNESS_BACKFILL_WINDOW_AHEAD` entries, with
+ * duplicates collapsed.
+ */
+import { describe, expect, it } from 'vitest';
+import type { Track } from './playerStore';
+import {
+  LOUDNESS_BACKFILL_WINDOW_AHEAD,
+  collectLoudnessBackfillWindowTrackIds,
+  isTrackInsideLoudnessBackfillWindow,
+} from './loudnessBackfillWindow';
+
+function track(id: string): Track {
+  return { id, title: id, artist: 'A', album: 'X', albumId: 'X', duration: 100 };
+}
+
+const big = Array.from({ length: 12 }, (_, i) => track(`t${i}`));
+
+describe('LOUDNESS_BACKFILL_WINDOW_AHEAD', () => {
+  it('is the value the runtime expects', () => {
+    expect(LOUDNESS_BACKFILL_WINDOW_AHEAD).toBe(5);
+  });
+});
+
+describe('isTrackInsideLoudnessBackfillWindow', () => {
+  it('matches the current track unconditionally', () => {
+    expect(isTrackInsideLoudnessBackfillWindow('t0', big, 0, big[0])).toBe(true);
+  });
+
+  it('matches an id inside the ahead window', () => {
+    // queueIndex 0, AHEAD 5 → indices 1..5 are inside, t3 must hit.
+    expect(isTrackInsideLoudnessBackfillWindow('t3', big, 0, big[0])).toBe(true);
+  });
+
+  it('returns false for an id beyond the ahead window', () => {
+    // From queueIndex 0, indices 1..5 inside → t6 (index 6) is outside.
+    expect(isTrackInsideLoudnessBackfillWindow('t6', big, 0, big[0])).toBe(false);
+  });
+
+  it('window slides with queueIndex', () => {
+    // queueIndex 4, AHEAD 5 → indices 5..9 are inside, t9 must hit, t10 must not.
+    expect(isTrackInsideLoudnessBackfillWindow('t9', big, 4, big[4])).toBe(true);
+    expect(isTrackInsideLoudnessBackfillWindow('t10', big, 4, big[4])).toBe(false);
+  });
+
+  it('returns false for empty queue', () => {
+    expect(isTrackInsideLoudnessBackfillWindow('t1', [], 0, null)).toBe(false);
+  });
+
+  it('returns false for empty trackId', () => {
+    expect(isTrackInsideLoudnessBackfillWindow('', big, 0, big[0])).toBe(false);
+  });
+
+  it('returns false when currentTrack is null and id is not in the queue window', () => {
+    expect(isTrackInsideLoudnessBackfillWindow('missing', big, 0, null)).toBe(false);
+  });
+});
+
+describe('collectLoudnessBackfillWindowTrackIds', () => {
+  it('returns current + next 5 entries', () => {
+    const ids = collectLoudnessBackfillWindowTrackIds(big, 0, big[0]);
+    expect(ids).toEqual(['t0', 't1', 't2', 't3', 't4', 't5']);
+  });
+
+  it('clamps the window to the end of the queue', () => {
+    const ids = collectLoudnessBackfillWindowTrackIds(big, 9, big[9]);
+    // queueIndex 9, AHEAD 5 → indices 10..11 available → t9, t10, t11
+    expect(ids).toEqual(['t9', 't10', 't11']);
+  });
+
+  it('omits the current track when null', () => {
+    const ids = collectLoudnessBackfillWindowTrackIds(big, 0, null);
+    expect(ids).toEqual(['t1', 't2', 't3', 't4', 't5']);
+  });
+
+  it('deduplicates when currentTrack is also in the ahead window', () => {
+    const queue = [track('a'), track('b'), track('a'), track('c')];
+    const ids = collectLoudnessBackfillWindowTrackIds(queue, 0, queue[0]);
+    expect(ids).toEqual(['a', 'b', 'c']);
+  });
+
+  it('returns just the current track for an empty queue', () => {
+    expect(collectLoudnessBackfillWindowTrackIds([], 0, track('only'))).toEqual(['only']);
+  });
+
+  it('returns an empty list when nothing is playing and the queue is empty', () => {
+    expect(collectLoudnessBackfillWindowTrackIds([], 0, null)).toEqual([]);
+  });
+});