nx_mesh_print_check.nx

nx_mesh_print_check.nx -- printability checks on the L1 canonical NxMesh (defined in nx_mesh.nx). Adds: - axis-aligned bounding box computation - watertight / 2-manifold edge check - tetrahedron factory (smallest non-degenerate closed mesh, for smokes and calibration tests) Composes nx_mesh.nx -- does NOT modify the L1 container. Per cardinal feedback-DRY-through-shared-libraries: extend at the primitive boundary, do not fork the canonical mesh. Why split from nx_mesh.nx: nx_mesh is the canonical L1 graphics primitive (used by nx_raster, nx_voxel_mesh, nx_render_pass). Manifold / bbox / printability are FDM-print-specific predicates. Keeping them in a sibling primitive prevents graphics callers from pulling print-only dependencies. Per cardinal NISHI_3D_PRINT_ROADMAP §2.1+2.2: a watertight, closed manifold mesh is a hard precondition for any slicer that claims to EXCEED OrcaSlicer's tree-support / adaptive-Z handling. Non-manifold meshes cascade into ambiguous slice contours. Detect at the boundary (Cardinal 12), not deep in the slicer. Coordinate convention: inherits nx_mesh's Q14 fixed-point xyz. 1 Q14 unit = 1/16384. Tetrahedron factory takes a unit-edge length and scales to Q14 internally. Manifold check algorithm (O(t²) for P0.1): In a watertight orientable 2-manifold, every undirected edge appears in EXACTLY two triangles, with opposite winding (one sees a->b, the other sees b->a). For each directed edge emitted by triangle T, find the reverse-direction edge in some OTHER triangle. Reverse count must equal 1 for every directed edge. O(t²) is fine for P0.1 smokes (tetrahedron = 4 tris); P0.2 swaps in a hashed-edge O(t) impl for the Christus (~200K tris). license_tier: ORIGINAL