fix(build): include the Music playlist in the static media sync

The Music-option tracks are referenced by app.js (MUSIC_PLAYLIST), not the
manifest/ring, so build_static skipped them — Music would 404 on the static
site. Add audio/music/<name>.mp3 explicitly. +1 pytest.

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

tools/build_static.py

@@ -151,6 +151,10 @@ def build_static(out_dir, media_out, *, media_base: str, app_path: str) -> dict:
     for s in ring.get("scales", []):
         if s.get("audio"):
             referenced.add(f"audio/{s['audio']}")
+    # The Music-option playlist is referenced by the client (app.js MUSIC_PLAYLIST),
+    # not the manifest/ring — add it explicitly or Music 404s on the static site.
+    for name in ("manatees", "dewdrop", "eastminster", "overheat", "concentration"):
+        referenced.add(f"audio/music/{name}.mp3")
     n = 0
     for rel in sorted(referenced):
         src = MEDIA_DIR / rel