Behavior-preserving prep for the new mix types. Extracts the three
inline candidate computations in BuildSystemPlaylists into
producers (produceForYou / produceSeedMixes / produceDiscover) and
drives the build off a systemPlaylistRegistry. The shared
machinery (run-claim guard, atomic delete+insert tx, post-commit
collages) is now generic over a []builtPlaylist.
Fatal-vs-skip error semantics unchanged: a base query failure
(PickTopPlayedTracksForUser, PickSeedArtists) still aborts the
whole build; candidate-load / per-seed-artist / Discover-bucket
failures are still logged and just yield fewer playlists.
Materialize order (for_you, songs_like_artist, discover) is
unchanged and functionally irrelevant.
No API/client/schema change — CI's system/foryou/service tests
verify For You / Songs-like-X / Discover parity. Adding a new mix
is now: a producer + one registry entry + its candidate query.
Next (R2): generic /api/playlists/system/{kind}/{refresh,shuffle}
off the registry; then the new kinds.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
bvandeusen
1379595e82