The #1249 mechanism (stamp WHY a track is in the snapshot at build
time, freeze it onto the play at ingestion, break it down in metrics)
generalizes from a For You one-off to the standard for every system
mix (#1270):
- Migration 0039 widens both pick_kind CHECKs (drop + re-add in the
same change) to taste/fresh + Discover's dormant/cross_user/random
+ tier1-3 for the rule-#131 eligibility ladders.
- GetForYouPickKindForTrack becomes GetSystemPickKindForTrack
(user, variant, track); ingestion stamps any systemPlaylistSources
play from its own variant's live snapshot, live + offline paths.
- Discover stamps its candidate bucket on discoverTrack before the
interleave, making the 40/30/30 allocation measurable; dedup keeps
the taking bucket's stamp.
- Metrics replace the for_you special-case with one pick-kind
vocabulary — any family with attributed plays gets a breakdown,
future stamping mixes need no metrics change.
- Web: breakdown sub-rows are now toggled per surface (collapsed by
default) so eight stamping mixes don't swamp the card.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TsF3cNoKrqCYsU78cXC8U6
bvandeusen
5faa57634b