any structural change (add / remove / fat-leaf escape) since the last rebuild. gates the dirty-gated rebuild in broadphase.optimize — a tree that no body has disturbed (e.g. a settled static field) stays clean and is never rebuilt.
the fat-leaf expansion margin (in world units) applied to each leaf AABB
rebuild marks the top N levels changed so they always re-partition
packed-SMI node topology, stride 5 (see file header)
packed-double node bounds, stride 6 (see file header)