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.
