feat(library): rating line, manga finish-flip notify, game links, dedupe

- render **Rating:** line in manga/book/game/comic body (after read/play
  status, gated on rating != '0' + stars non-empty), matching watchlist's
  render.ts convention
- manga finish-flip now notifies user (final chapters out), matching
  chapter-flip's existing notify path
- game renderGame emits ## Links (Steam by appid, RAWG by url) — fixes
  latent owned-heading strip where user-added Links sections silently
  vanished since games never re-rendered the heading
- move duplicated deriveReadStatus/deriveRating/parseNumOrNull into
  library/convert.ts, import across specs; zero behavior change

tests/fixtures/canonical-game.md

@@ -24,6 +24,7 @@ tags: [games, game]
 **Game** · 2018-03-02 · Metacritic 79
 
 **Play Status:** Played
+**Rating:** ⭐️⭐️⭐️⭐️ (4/5)
 
 ## Synopsis
 From the creators of Human Resource Machine! Program a workforce of dumb humans to do your bidding in this fiendish puzzle game.
@@ -32,5 +33,8 @@ From the creators of Human Resource Machine! Program a workforce of dumb humans
 **Publisher:** Tomorrow Corporation
 **Platforms:** PC
 
+## Links
+- [Steam](https://store.steampowered.com/app/792100/)
+
 ## My Notes