refactor(player): M0 — extract pure helpers from playerStore.ts (#554)

Moves four self-contained helpers into src/utils/, each with co-located
characterization tests. playerStore re-exports them for the ~30 existing
call sites; Phase E will migrate those imports.

  - shuffleArray              (Fisher-Yates, generic)
  - resolveReplayGainDb       (track/album/auto mode resolution)
  - songToTrack               (Subsonic -> Track shape)
  - buildInfiniteQueueCandidates  (Instant-Mix top-up source)

playerStore.ts: 3732 -> 3618 LOC (-114).

src/utils/buildInfiniteQueueCandidates.ts

@@ -0,0 +1,61 @@
+import { getRandomSongs, getSimilarSongs2, getTopSongs } from '../api/subsonic';
+import {
+  enrichSongsForMixRatingFilter,
+  getMixMinRatingsConfigFromAuth,
+  passesMixMinRatings,
+} from './mixRatingFilter';
+import { shuffleArray } from './shuffleArray';
+import { songToTrack } from './songToTrack';
+import type { Track } from '../store/playerStore';
+
+/**
+ * Infinite queue source strategy (Instant Mix-like):
+ * 1) Prefer artist-driven candidates (Top + Similar) around the current track.
+ * 2) Fallback to random songs when artist-driven fetches are empty.
+ */
+export async function buildInfiniteQueueCandidates(
+  seedTrack: Track | null,
+  existingIds: Set<string>,
+  count = 5,
+): Promise<Track[]> {
+  const RANDOM_TOPUP_BATCH_SIZE = Math.max(10, count * 2);
+  const RANDOM_TOPUP_MAX_BATCHES = 8;
+  const artistId = seedTrack?.artistId?.trim() || null;
+  const artistName = seedTrack?.artist?.trim() || null;
+
+  const [similar, top] = await Promise.all([
+    artistId ? getSimilarSongs2(artistId).catch(() => []) : Promise.resolve([]),
+    artistName ? getTopSongs(artistName).catch(() => []) : Promise.resolve([]),
+  ]);
+
+  const seedId = seedTrack?.id ?? null;
+  const mixCfg = getMixMinRatingsConfigFromAuth();
+  const mixedSources = [...top, ...similar];
+  const filteredMixedSongs = mixCfg.enabled
+    ? (await enrichSongsForMixRatingFilter(mixedSources, mixCfg)).filter(s => passesMixMinRatings(s, mixCfg))
+    : mixedSources;
+  const out: Track[] = shuffleArray(
+    filteredMixedSongs
+      .map(songToTrack)
+      .filter(t => t.id !== seedId && !existingIds.has(t.id)),
+  )
+    .slice(0, count)
+    .map(t => ({ ...t, autoAdded: true as const }));
+
+  const seenIds = new Set<string>([...existingIds, ...out.map(t => t.id)]);
+  for (let b = 0; out.length < count && b < RANDOM_TOPUP_MAX_BATCHES; b++) {
+    const random = await getRandomSongs(RANDOM_TOPUP_BATCH_SIZE, seedTrack?.genre).catch(() => []);
+    if (!random.length) break;
+    const filteredRandomSongs = mixCfg.enabled
+      ? (await enrichSongsForMixRatingFilter(random, mixCfg)).filter(s => passesMixMinRatings(s, mixCfg))
+      : random;
+    for (const track of shuffleArray(filteredRandomSongs.map(songToTrack))) {
+      if (track.id === seedId || seenIds.has(track.id)) continue;
+      out.push({ ...track, autoAdded: true as const });
+      seenIds.add(track.id);
+      if (out.length >= count) break;
+    }
+  }
+
+  return out.slice(0, count);
+}