refactor(player): E.17 — extract stream-cache-to-hot-cache promoter (#580)

`promoteCompletedStreamToHotCache` — wraps the `promote_stream_cache_to_hot_cache`
Rust IPC, forwards the resolved path + size into `useHotCacheStore` as a
`'stream-promote'` entry — moves into `src/store/promoteStreamCache.ts`.
File-private with four call sites; no caller-side changes outside
playerStore's own import.

9 focused tests pin the payload shape (incl. the `'mp3'` suffix fallback
and the null customDir pass-through), the success path that records the
entry, the early-returns for null / empty path, the `size || 0` fallback,
and the silent error swallow.

playerStore 3207 → 3189 LOC.

src/store/playerStore.ts

@@ -104,6 +104,7 @@ import {
   setBytePreloadingId,
   setGaplessPreloadingId,
 } from './gaplessPreloadState';
+import { promoteCompletedStreamToHotCache } from './promoteStreamCache';
 
 // Re-export so TauriEventBridge + persistence test keep their existing
 // `from './playerStore'` imports.
@@ -807,25 +808,6 @@ function prefetchLoudnessForEnqueuedTracks(
   }
 }
 
-async function promoteCompletedStreamToHotCache(track: Track, serverId: string, customDir: string | null) {
-  try {
-    const res = await invoke<{ path: string; size: number } | null>(
-      'promote_stream_cache_to_hot_cache',
-      {
-        trackId: track.id,
-        serverId,
-        url: buildStreamUrl(track.id),
-        suffix: track.suffix || 'mp3',
-        customDir,
-      },
-    );
-    if (!res || !res.path) return;
-    useHotCacheStore.getState().setEntry(track.id, serverId, res.path, res.size || 0, 'stream-promote');
-  } catch {
-    // best-effort promotion; normal hot-cache prefetch remains fallback
-  }
-}
-
 // ─── Audio event handlers (called from initAudioListeners) ───────────────────
 
 function handleAudioPlaying(_duration: number) {