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/promoteStreamCache.ts

@@ -0,0 +1,39 @@
+import { invoke } from '@tauri-apps/api/core';
+import { buildStreamUrl } from '../api/subsonic';
+import { useHotCacheStore } from './hotCacheStore';
+import type { Track } from './playerStore';
+
+/**
+ * Promote a track whose stream cache is full to the on-disk hot cache.
+ * Rust copies the cached bytes into the hot-cache directory and returns
+ * the resolved path + size; the JS-side `useHotCacheStore` index gets the
+ * entry tagged `'stream-promote'` so the LRU treats it the same as a
+ * prefetch hit.
+ *
+ * Best-effort: any failure is swallowed because the regular hot-cache
+ * prefetch path remains a fallback. `customDir` may be null when the user
+ * hasn't picked a hot-cache directory yet — Rust then writes to the
+ * default location.
+ */
+export async function promoteCompletedStreamToHotCache(
+  track: Track,
+  serverId: string,
+  customDir: string | null,
+): Promise<void> {
+  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
+  }
+}