perf(v4.2): remove useless BVH build in LargeAssemblyManager

build_index was converting 10K AABBs → 120K triangles → BVH build (26s),
but query_visible always used linear scan over part_aabbs_. BVH never queried.

10K AABB linear intersect: <1ms. Build time: 31s → 1s.

src/brep/large_assembly.cpp

@@ -90,41 +90,6 @@ void LargeAssemblyManager::build_index(const Assembly& assembly) {
 
     traverse(assembly.root, Transform3D::Identity());
 
-    // Build BVH from AABBs → convert to triangles
-    std::vector<Triangle3D> tris;
-    for (const auto& bb : part_aabbs_) {
-        Point3D mn = bb.min(), mx = bb.max();
-        // 12 triangles for the box faces
-        std::vector<Point3D> corners = {
-            mn, Point3D(mx.x(), mn.y(), mn.z()), Point3D(mx.x(), mx.y(), mn.z()),
-            Point3D(mn.x(), mx.y(), mn.z()),
-            Point3D(mn.x(), mn.y(), mx.z()), Point3D(mx.x(), mn.y(), mx.z()),
-            mx, Point3D(mn.x(), mx.y(), mx.z())
-        };
-        // bottom
-        tris.push_back(Triangle3D(corners[0], corners[2], corners[1]));
-        tris.push_back(Triangle3D(corners[0], corners[3], corners[2]));
-        // top
-        tris.push_back(Triangle3D(corners[4], corners[5], corners[6]));
-        tris.push_back(Triangle3D(corners[4], corners[6], corners[7]));
-        // front
-        tris.push_back(Triangle3D(corners[0], corners[1], corners[5]));
-        tris.push_back(Triangle3D(corners[0], corners[5], corners[4]));
-        // back
-        tris.push_back(Triangle3D(corners[2], corners[3], corners[7]));
-        tris.push_back(Triangle3D(corners[2], corners[7], corners[6]));
-        // left
-        tris.push_back(Triangle3D(corners[0], corners[4], corners[7]));
-        tris.push_back(Triangle3D(corners[0], corners[7], corners[3]));
-        // right
-        tris.push_back(Triangle3D(corners[1], corners[2], corners[6]));
-        tris.push_back(Triangle3D(corners[1], corners[6], corners[5]));
-    }
-
-    if (!tris.empty()) {
-        bvh_.build(tris);
-    }
-
     part_count_ = static_cast<int>(part_names_.size());
     index_built_ = true;
 }