Image split into 9 tiles, driven by the rotating‑squares
auxetic mechanism — pitch l(θ) = a(cos θ + sin θ) —
folds 0° → 90°, settling re‑aligned (tiles turned ±90°)
→ negative Poisson's ratio ν = −1
Bundled sample image by default — append ?img=URL to
animate any picture (data: URIs work too); ?pattern=1
for the plain pattern.