Mesh geometry import

[Luochenghuang]

Summary

Closes #67. Adds a new MESH geometric object type for importing arbitrary triangulated 3D surfaces (e.g., from STL files) into libctl's geometry system.

Design

The mesh follows the same tagged-union dispatch as SPHERE, CYLINDER, BLOCK, and PRISM. A flat-array BVH with SAH binning accelerates all queries to O(log N).

All seven core operations are implemented:

Operation	Implementation
point_in_mesh	Ray casting with lazy 3-ray majority vote (Moller-Trumbore + BVH)
normal_to_mesh	BVH nearest-face search (Eberly closest-point-on-triangle)
intersect_line_segment_with_mesh	Sorted ray-mesh intersections with even-odd parity accumulation
get_mesh_volume	Divergence theorem (signed tetrahedron volumes)
get_mesh_bounding_box	Cached from BVH root node
display_mesh_info	Vertex/face counts and closure status
init_mesh	Validates, computes normals, checks closure, fixes winding, builds BVH

Meep's subpixel smoothing works automatically. No meep C++ changes required.

API

// Closed triangulated 3D mesh. The mesh must be watertight (every edge
// shared by exactly 2 faces); open meshes are detected and produce a warning.
geometric_object make_mesh(material_type material,
                           const vector3 *vertices, int num_vertices,
                           const int *triangles, int num_triangles);

geometric_object make_mesh_with_center(material_type material, vector3 center,
                                       const vector3 *vertices, int num_vertices,
                                       const int *triangles, int num_triangles);

Robustness

• Lazy majority vote: casts 1 ray; only falls back to 3-ray vote when deduplication detects a shared-edge hit. Correct and fast for the common case.
• Mesh closure validation: edge-adjacency hash table detects boundary and non-manifold edges. point_in_mesh returns false for open meshes.
• reinit_mesh guard: skips redundant BVH rebuilds in geom_fix_object_ptr (mesh uses absolute coordinates, not lattice-relative).

Test plan

• 16 unit tests: point-in, volume, bbox, normals, line-segment intersection, copy/destroy, closure detection (open, boundary edge, non-manifold, isolated vertex)
• Cube mesh vs Block primitive: 10K random points, 0 mismatches

[stevengj]

This should be 1e-15 * lengthscale where lengthscale is some characteristic scale, e.g. the diameter of the whole objects.

Similarly, parent_area = 1e-30 * lengthscale * lengthscale.

[stevengj]

These "magic small numbers" should be relative to a lengthscale, e.g. the diameter of the object.

[stevengj]

Would be good to document that $n = (v_1 - v_0) \times (v_2 - v_0)$ should all point outwards or all point inwards on input (i.e. the faces should be oriented consistently), and that internally it normalizes them to all point outwards.