librosa's peak-picker finds peaks in whatever onset envelope it is handed, flat ones
included: a steady 50 Hz sine yields 84 onsets over six seconds (14/s) and was being
filed as drums, which made the bass branch unreachable for exactly the signal it was
written for. Onset count is now gated on the frame-RMS crest — ~1.0 for a sustained
tone against 3.1-9.5 for anything struck.

Measured across MAREA/ANGIE/ME to pick the threshold rather than guess it. Worth
recording that crest is a SPARSITY measure, not a percussion one — MAREA's single
vocal phrase reads 17.98 — so it only ever gates onset density and is never used
alone. HPSS was tried first and rejected: a kick-heavy drum bus reads 0.094
percussive, because HPSS hears a modern kick as harmonic.

18 tests.