feat: optional direct Bandcamp/Apple/YouTube links via Odesli (flag-gated) (closes task/odesli-direct-cross-platform-links)

Source: task/odesli-direct-cross-platform-links (plan/improvement-backlog) — bandcamp was only a Google search; Odesli gives direct cross-platform links for Spotify-resolved names.

- service/odesli.js: makeOdesli({fetch,baseUrl}) -> getLinks(spotifyUrl); pure parseOdesli core; bounded AbortSignal.timeout; non-200/timeout/garbage -> {} (graceful).
- resolver.js: opt-in ODESLI / ODESLI_URL flag (default OFF). When ON, enrich each Spotify-resolved result with additive bandcamp/appleMusic/youtube fields (isHttpUrl-validated), cached per Spotify url. OFF -> no call, byte-identical behavior. /health + startup log surface the flag.
- extension: a tiny additive "bc" link to a direct Bandcamp page when result.bandcamp is present (reuses isSafeHttpUrl); primary artist/album/spotify/google rendering unchanged. styles.css: bandcamp-teal secondary link.
- odesli.test.js + odesli.fixture.json: parseOdesli unit-tested against a real captured Odesli response (Blood Incantation album -> direct bandcamp; absent apple/youtube omitted; junk -> {}); getLinks 200/non-200/throw/parse-error paths.
- docs: .env.example, README, AGENTS.md note the opt-in flag.

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

extension/styles.css

@@ -16,6 +16,16 @@
 .rsl-google {
   text-decoration-color: #4a72d0 !important; /* google-ish blue */
 }
+/* Optional secondary "bc" link to a DIRECT Bandcamp page (the Odesli enrichment). Small and
+   superscript so it sits unobtrusively beside the primary artist/album link. */
+.rsl-bandcamp {
+  text-decoration-color: #629aa9 !important; /* bandcamp teal */
+}
+.rsl-direct {
+  font-size: 0.72em;
+  vertical-align: super;
+  margin-left: 2px;
+}
 .rsl-link:hover {
   text-decoration-style: solid !important;
 }