PLN review pass:
- tooltip layer: hover any mark for the tracks/examples behind it. cloud dots
  (track + bpm + date), tempo line dots (median + what it means), histogram
  bands, gig/sample columns, family segments (count + % + example members),
  idiom bars (plain-language gloss of each phrase), staple bars, collab dots
- collab story now plots EVERY track as a dot; tracks at the same BPM pile into
  one larger dot (radius ~ count); median drawn as a tick; sample-tell chips get
  lift tooltips. needs tide_eda collab_fingerprint to emit per-track {name,bpm}
- copy: removed cheap 'not X, Y' contrasts + em dashes per impeccable clarify;
  added 'hover anything' affordance hints