A seeded 8-bar 4/4 pattern at 120 BPM (sr=22050) puts the accented hit — a loud 60Hz kick +
noise burst — on beat index 1 of every bar, with quiet hats elsewhere. The true beat grid
(every 0.5s) plus audio goes into findDownbeatPhase(y, beats, 120, sr).
Proofs (node-verified): phase === 1 with a decisive score margin, one downbeat
per bar (8 total), and findFirstDownbeat on a signal with one silent leading bar
returns 2.000s exactly (±0.25s asserted). Repairs proven here (2026-07-02): phase
rounding wraps 4→0 instead of silently discarding, and accent strength is measured
forward from the (uncentered, frame-start) onset time — the old centered window read pure
silence on sparse material and scored every phase 0. The play button thumps on each reported
downbeat so the "boom on 1" is audible.