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.ts

@@ -0,0 +1,48 @@
+import type { Track } from './playerStore';
+
+/**
+ * After a bulk enqueue (queue replace, append-many, lucky-mix) the runtime
+ * warms the loudness cache for the current track + the next N entries so
+ * the engine's `audio_chain_preload` sees a real cached gain instead of
+ * the startup trim. These helpers compute that window — both as a
+ * "does this id sit inside it?" predicate and as the explicit id list
+ * the prefetch loop iterates over.
+ *
+ * Pure functions of the state slice — no store imports, no side effects.
+ * The caller passes the queue + index + current track so the test surface
+ * stays trivial and there's no top-level coupling back to playerStore.
+ */
+export const LOUDNESS_BACKFILL_WINDOW_AHEAD = 5;
+
+export function isTrackInsideLoudnessBackfillWindow(
+  trackId: string,
+  queue: Track[],
+  queueIndex: number,
+  currentTrack: Track | null,
+): boolean {
+  if (!trackId) return false;
+  if (currentTrack?.id === trackId) return true;
+  if (queue.length === 0) return false;
+  const start = Math.max(0, queueIndex + 1);
+  const end = Math.min(queue.length, start + LOUDNESS_BACKFILL_WINDOW_AHEAD);
+  for (let i = start; i < end; i++) {
+    if (queue[i]?.id === trackId) return true;
+  }
+  return false;
+}
+
+export function collectLoudnessBackfillWindowTrackIds(
+  queue: Track[],
+  queueIndex: number,
+  currentTrack: Track | null,
+): string[] {
+  const ids = new Set<string>();
+  if (currentTrack?.id) ids.add(currentTrack.id);
+  const start = Math.max(0, queueIndex + 1);
+  const end = Math.min(queue.length, start + LOUDNESS_BACKFILL_WINDOW_AHEAD);
+  for (let i = start; i < end; i++) {
+    const tid = queue[i]?.id;
+    if (tid) ids.add(tid);
+  }
+  return Array.from(ids);
+}