Marching squares over each cluster's own contribution to the shared
blurred bitmap, recovered by projecting premultiplied pixels onto the
cluster's paint color (blends split proportionally between overlapping
halos — the natural divider). Boundary level = fraction of the
cluster's peak field, user-adjustable in the settings panel; 0 = off.
One paint, one blur, one readback; contouring is cheap typed-array
math over small per-cluster crops (stride-2 sampling: measured 1.2ms
for all 26 clusters, vs 7.3ms naive). During the live sim boundaries
recompute every 2nd region rebuild and stale paths ride along — drag
cadence matches the pre-boundary baseline (avg 8.46ms vs 8.37ms).