X is a 2-dim ramp (40 frames); Y duplicates frames 10–19 (known warp map).
The heatmap shows the accumulated-cost matrix D with the recovered warping
path overlaid — it must ride the zero-cost valley exactly: total cost 0,
every path pair obeying the duplication map, and subsequence DTW pinning an
embedded copy at its exact offset. Same asserts as
examples/node/dtw.mjs (node-verified).