feat: expose vip flag on Song for trial-only detection

Add optional vip?: boolean to the Song interface so downstream clients
(e.g. PowerfulTS) can mark copyright-restricted songs that non-VIP users
can only play as a trial fragment, before playback starts.

- provider.ts: add optional vip?: boolean (backward compatible)
- netease.ts: extract mapNeteaseSongs() pure fn; map fee to vip
  (1=VIP, 4=album-only). fee=8 (free low-quality) is excluded because
  it plays in full, just at lower quality.
- qq.ts: mapQqSongs() maps pay.payplay/paytrackprice to vip (one fix
  covers all callers); getDailyRecommendSongs inline mapping too.
- tests: vip mapping assertions for netease fee (1/4=vip, 0/8=free) and
  qq pay fields.

src/music/qq.test.ts

@@ -22,10 +22,24 @@ describe("QQ adapter", () => {
         duration: 243,
         coverUrl: "https://y.gtimg.cn/music/photo_new/T002R300x300M000alb001.jpg",
         platform: "qq",
+        vip: false,
       },
     ]);
   });
 
+  it("mapQqSongs maps pay field to vip flag", () => {
+    const out = mapQqSongs([
+      { mid: "v1", name: "VIP playplay", singer: [], album: {}, interval: 100, pay: { payplay: 1, paytrackprice: 0 } },
+      { mid: "v2", name: "VIP trackprice", singer: [], album: {}, interval: 100, pay: { payplay: 0, paytrackprice: 1 } },
+      { mid: "f1", name: "Free", singer: [], album: {}, interval: 100, pay: { payplay: 0, paytrackprice: 0 } },
+      { mid: "f2", name: "No pay field", singer: [], album: {}, interval: 100 },
+    ]);
+    expect(out[0].vip).toBe(true);
+    expect(out[1].vip).toBe(true);
+    expect(out[2].vip).toBe(false);
+    expect(out[3].vip).toBe(false);
+  });
+
   it("mapQqAlbums maps albumMID-style raw entries", () => {
     const raw = [
       {