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/radioSessionState.ts

@@ -0,0 +1,66 @@
+/**
+ * Per-session bookkeeping for the radio-feature (auto-mix tail of
+ * tracks based on a seed artist):
+ *
+ *  - **radioFetching** — concurrent-fetch guard. Stops two parallel
+ *    `getSimilarSongs` / `getTopSongs` requests from racing each other
+ *    when both `next()` and the proactive top-up path fire close
+ *    together.
+ *
+ *  - **currentRadioArtistId** — the seed artist that started the
+ *    current radio session. Survives track advances so subsequent
+ *    top-ups can resolve a new tail even when the now-playing track
+ *    has no `artistId` of its own.
+ *
+ *  - **radioSessionSeenIds** — every id the current radio session has
+ *    enqueued so far, *including* entries that were trimmed off the
+ *    front of the queue once it grew past `HISTORY_KEEP`. Without this
+ *    set, the queue's own id-set wasn't enough to dedupe: a song
+ *    played 8 tracks ago is gone from the queue and the next
+ *    Last.fm / topSongs response could re-add it (issue #500). Reset
+ *    on `setRadioArtistId(other)` and on `clearQueue()`.
+ */
+
+let radioFetching = false;
+let currentRadioArtistId: string | null = null;
+let radioSessionSeenIds = new Set<string>();
+
+export function isRadioFetching(): boolean {
+  return radioFetching;
+}
+
+export function setRadioFetching(value: boolean): void {
+  radioFetching = value;
+}
+
+export function getCurrentRadioArtistId(): string | null {
+  return currentRadioArtistId;
+}
+
+export function setCurrentRadioArtistId(id: string | null): void {
+  currentRadioArtistId = id;
+}
+
+export function hasRadioSessionSeen(id: string): boolean {
+  return radioSessionSeenIds.has(id);
+}
+
+export function addRadioSessionSeen(id: string): void {
+  radioSessionSeenIds.add(id);
+}
+
+export function deleteRadioSessionSeen(id: string): void {
+  radioSessionSeenIds.delete(id);
+}
+
+/** Drop every id the current session has remembered — call when the seed artist changes or the queue is cleared. */
+export function clearRadioSessionSeenIds(): void {
+  radioSessionSeenIds = new Set();
+}
+
+/** Test-only: reset all three pieces of state. */
+export function _resetRadioSessionStateForTest(): void {
+  radioFetching = false;
+  currentRadioArtistId = null;
+  radioSessionSeenIds = new Set();
+}