Dynamic time warping is how you line up two takes of the same material
when one of them drags or rushes — it stretches time until the two
sequences match. The heatmap shows the accumulated-cost matrix D with the
recovered warping path overlaid, riding the cheapest valley. Technically:
X is a 2-dim ramp (40 frames) and Y duplicates frames 10–19 (known warp
map), so the path 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).