A metronome-steady click track goes in, and the beat tracker has to find the tempo and mark
every beat. Orange ticks show what it detected, and the play button adds a beep on each
detected beat so you can hear the lock. Technically: a synthetic 10s click train at 120 BPM
(5ms decaying 1kHz bursts every 0.5s, sr=22050) goes through the canonical
beat_track() and the explicit quick tier quickTempo(). Proofs: tempo
lands in one of the two lag bins bracketing 120 BPM (117.45 / 123.05 at hop=512), 18–21
beats over 10s, median inter-beat interval within one hop (23.2ms) of 0.500s, and quickTempo
lands in the SAME lag bin with measured confidence > 0. Same asserts as
examples/node/beat-tracker.mjs.