librosa.sequence.dtw's default step sizes permit pure horizontal/vertical
moves; on songs whose chroma is self-similar for long stretches the flat
cost surface let the path collapse (minutes of score onto one audio frame),
so auto-sync produced monotonic-but-garbage sync points and the per-bar
warp imported charts badly out of sync while reporting success.
Use the standard music-sync step pattern [[1,1],[1,2],[2,1]] (local tempo
ratio bounded to 0.5x-2x), falling back to unconstrained steps if the
global length ratio makes it infeasible.
Validated on the reported song (138 BPM tab, YouTube audio): coarse points
now track 1:1, refine holds slopes 0.77-1.04, warped downbeats hit onset
peaks at 3.3x background energy.
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>