refactor(playback): move the audio engine into features/playback

Relocate the playback/queue/transport/audio-output engine out of the type-first
store/ + utils/playback/ + utils/audio/ dirs into a cohesive src/features/playback/,
structure-preserving:
  store/<x>                    -> features/playback/store/<x>
  store/audioListenerSetup/<x> -> features/playback/store/audioListenerSetup/<x>
  utils/playback/<x>           -> features/playback/utils/playback/<x>
  utils/audio/<x>              -> features/playback/utils/audio/<x>

184 files moved (107 source + 77 tests), 365 consumers rewritten. Pure move — no
behavior change, no state-split (the playerStore state-split stays a separate M5
question). Enabled by this session's decouple seams (artist/offline/orbit/auth →
core registries), so the engine carries no inbound core->feature inversion: store/
now holds only the 50 cross-cutting global stores (auth family, the seams, library
index, UI/settings stores).

KEPT OUT of the move (would re-create global->engine edges): the 3 pure config
helpers utils/audio/{loudnessPreAnalysisSlider,hiResCrossfadeResample} +
utils/playback/autodjOverlapCap (authStore + settings UI read them — they stay in
utils/). Ambiguous view-state stores (eqStore, queueToolbarStore,
playerBarLayoutStore) stay global (no engine imports).

Consumers use DEEP paths (@/features/playback/...), no barrel — matches the lib/
approach and avoids barrel-mock-collapse across the 140 usePlayerStore consumers.
Two tolerated type-only core->feature edges remain (localPlaybackStore->QueueItemRef,
localPlaybackMigration->HotCacheEntry, both erased).

tsc 0, lint 0, full suite 319/2353 green, iron-rule clean (no runtime store->feature
import). Behavior-touching only via the prerequisite bridge seam (already QA-flagged);
the move itself is pure.

src/features/playback/store/waveformRefresh.ts

@@ -0,0 +1,73 @@
+import { invoke } from '@tauri-apps/api/core';
+import { coerceWaveformBins } from '@/utils/waveform/waveformParse';
+import { getPlaybackIndexKey } from '@/features/playback/utils/playback/playbackServer';
+import { usePlayerStore } from '@/features/playback/store/playerStore';
+import { getWaveformRefreshGen } from '@/features/playback/store/waveformRefreshGen';
+
+/** Subsonic-server waveform-cache row as Rust hands it back. */
+export type WaveformCachePayload = {
+  /** May be `number[]` or `Uint8Array` depending on Tauri IPC / serde path. */
+  bins: number[] | Uint8Array;
+  binCount: number;
+  isPartial: boolean;
+  knownUntilSec: number;
+  durationSec: number;
+  updatedAt: number;
+};
+
+/**
+ * Fetch the cached waveform row for `trackId` from Rust and apply its bins
+ * to the player store — but only if (a) the refresh generation snapshot
+ * still matches (no newer invalidation has fired meanwhile) and (b) the
+ * track is still the current one. Best-effort: any failure leaves the
+ * seekbar with the placeholder waveform.
+ */
+/**
+ * Fetch a track's cached waveform bins **without touching the player store** —
+ * used by the silence-aware crossfade to inspect the *next* track's leading
+ * silence while a different track is still playing (writing `waveformBins` here
+ * would replace the current track's seekbar). Returns `null` on a cold miss /
+ * any failure so callers degrade to no-trim.
+ */
+export async function fetchWaveformBins(
+  trackId: string,
+  serverId?: string | null,
+): Promise<number[] | null> {
+  if (!trackId) return null;
+  try {
+    const row = await invoke<WaveformCachePayload | null>('analysis_get_waveform_for_track', {
+      trackId,
+      serverId: serverId ?? getPlaybackIndexKey() ?? null,
+    });
+    const bins = row ? coerceWaveformBins(row.bins) : null;
+    return bins && bins.length > 0 ? bins : null;
+  } catch {
+    return null;
+  }
+}
+
+export async function refreshWaveformForTrack(trackId: string): Promise<void> {
+  if (!trackId) return;
+  const gen = getWaveformRefreshGen(trackId);
+  try {
+    const row = await invoke<WaveformCachePayload | null>('analysis_get_waveform_for_track', {
+      trackId,
+      serverId: getPlaybackIndexKey() || null,
+    });
+    if (getWaveformRefreshGen(trackId) !== gen) return;
+    // Never apply bins for a non-current track (e.g. gapless byte-preload fetches the neighbour).
+    if (usePlayerStore.getState().currentTrack?.id !== trackId) return;
+    const bins = row ? coerceWaveformBins(row.bins) : null;
+    if (!bins || bins.length === 0) {
+      usePlayerStore.setState({
+        waveformBins: null,
+      });
+      return;
+    }
+    usePlayerStore.setState({
+      waveformBins: bins,
+    });
+  } catch {
+    // best-effort; seekbar falls back to placeholder waveform
+  }
+}