refactor(player): E.5 — extract playback-URL routing into module (#568)

Three file-private helpers + their shared module-scoped track id move
into src/store/playbackUrlRouting.ts: recordEnginePlayUrl,
playbackSourceHintForResolvedUrl, shouldRebindPlaybackToHotCache. The
`lastOpenedWithHttpTrackId` mutable goes with them — only those three
read it. No external callers, no re-exports needed.

Adds 12 focused tests covering the source-kind classifier
(stream / hot / offline), the rebind decision across stream:-prefix
forms, the empty-serverId / un-recorded edge cases, and the
test-only reset helper.

playerStore 3490 → 3470 LOC.

src/store/playbackUrlRouting.ts

@@ -0,0 +1,39 @@
+import { resolvePlaybackUrl, type PlaybackSourceKind } from '../utils/resolvePlaybackUrl';
+import { sameQueueTrackId } from '../utils/queueIdentity';
+import { useOfflineStore } from './offlineStore';
+
+/**
+ * Helpers that classify the URL `audio_play` was last invoked with, so the
+ * runtime can rebind playback onto a freshly-promoted hot-cache entry the
+ * next time the same track plays.
+ *
+ * `lastOpenedWithHttpTrackId` remembers the most recent track id we asked
+ * the engine to fetch over HTTP (anything that isn't the `psysonic-local://`
+ * scheme). When the hot-cache later promotes that track to a local URL we
+ * compare against this id to decide whether a transparent rebind is worth
+ * triggering.
+ */
+let lastOpenedWithHttpTrackId: string | null = null;
+
+export function recordEnginePlayUrl(trackId: string, url: string): void {
+  lastOpenedWithHttpTrackId = url.startsWith('psysonic-local://') ? null : trackId;
+}
+
+/** Matches `playTrack` / PlayerBar: stream vs hot-cache vs offline file from resolved `audio_play` URL. */
+export function playbackSourceHintForResolvedUrl(trackId: string, serverId: string, url: string): PlaybackSourceKind {
+  if (!url.startsWith('psysonic-local://')) return 'stream';
+  return useOfflineStore.getState().getLocalUrl(trackId, serverId) ? 'offline' : 'hot';
+}
+
+export function shouldRebindPlaybackToHotCache(trackId: string, serverId: string): boolean {
+  if (!serverId) return false;
+  if (!lastOpenedWithHttpTrackId || !sameQueueTrackId(lastOpenedWithHttpTrackId, trackId)) {
+    return false;
+  }
+  return resolvePlaybackUrl(trackId, serverId).startsWith('psysonic-local://');
+}
+
+/** Test-only: clear the module-scoped track id so each test starts clean. */
+export function _resetPlaybackUrlRoutingForTest(): void {
+  lastOpenedWithHttpTrackId = null;
+}