nx_bvh.nx

nx_bvh.nx -- axis-aligned bounding volume hierarchy on NxMesh triangles. Top-down median-split build, flat node array. Used by the slicer (plane vs triangle pruning), by support tree generation (ray casts for overhang detection), by mesh repair (find near-duplicate verts via spatial proximity). Composes nx_mesh -- does NOT modify it. Per Cardinal 9 (single responsibility): BVH is a spatial index ON nx_mesh, not part of the mesh itself. Storage: - Flat array of NxBvhNode (64 bytes each) - Per-tri precomputed centroid + AABB in NxBvhTriInfo - Permutation array tri_order: leaf nodes point into tri_order; tri_order[i] is the mesh triangle index Build complexity: O(n log n) expected with median-of-three partition. Real-world worst case on degenerate meshes (all tris colinear on the chosen axis): O(n^2). Acceptable for P0.2b smoke; the Christus is well-distributed and will see expected complexity. Node layout invariant: when an internal node is created, its LEFT child is the very next node in the array (idx + 1) and its RIGHT child is whatever node is allocated after the left subtree completes. Depth-first build maintains this. Per cardinal NISHI_3D_PRINT_ROADMAP §2.7: this BVH IS the incremental-slice substrate. Re-slicing on a config change with the same mesh reuses the same BVH; only the slice traversal reruns. That's the slice-time win lever vs OrcaSlicer's per- param-change full re-slice. license_tier: ORIGINAL