docs(CLAUDE): document the single-track metadata resolver (Deezer→MusicBrainz→clean fallback)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -105,6 +105,17 @@ Backend runs in Docker (see `docker-compose.yml`); on the deploy host it lives a
 - **Duplicate detection** (`metadata.js findExistingTrack`): searches case-insensitively across
   all album folders under the matching artist folder; compares normalized artist+title ignoring
   track prefix and release/variant suffix. Handles DJ OKAWARI vs DJ Okawari and "- Single" drift.
+- **Single-track metadata resolver = `backend/lib/trackmeta.js`** (Deezer → MusicBrainz → clean
+  fallback). Deezer's `pickTrackMatch` requires an **artist correspondence** (equal, or one name
+  contained in the other after stripping non-alphanumerics) — the old blind *title-only* fallback
+  grabbed a same-titled song by a different artist (e.g. "Memories" by Rab Noakes for "☆NoAki☆") and
+  applied its wrong album/cover/year. On no match it returns `{}`; then `musicbrainz.js` (rate-limited
+  ~1/s, `User-Agent` required, Cover Art Archive for covers, **same** artist discipline) is tried; if
+  that also misses, the download's built-in clean fallback runs (**album_artist = song artist, album =
+  "Singles", cover = source thumbnail, year/genre left untouched**). Only the three *single-track*
+  call sites use `trackmeta`; the album batch stays Deezer-only (avoids MB's rate limit). The clean
+  fallback always existed but was unreachable while the match was too greedy. Matching is unit-tested
+  (`deezer.test.js`, `musicbrainz.test.js`).
 - **Known limitation:** yt-dlp's YTM/YouTube *search* can't surface very obscure Art Tracks by
   ambiguously-named artists (e.g. "MINA") — the browser finds them, yt-dlp's API extraction
   doesn't (confirmed across params/clients/deno). For those, the **direct link in the importer**