fix: shell vertex ID→map lookup, fix SEGFAULT

src/brep/modeling.cpp

@@ -1,5 +1,6 @@
 #include "vde/brep/modeling.h"
 #include <cmath>
+#include <map>
 #include <algorithm>
 #include <cassert>
 
@@ -746,14 +747,16 @@ BrepModel shell(const BrepModel& body, int open_face, double thickness) {
     BrepModel result;
 
     // ── 1. Offset all vertices inward ──
-    std::vector<int> outer_vid(body.num_vertices());  // map old id → new outer id
-    std::vector<int> inner_vid(body.num_vertices());  // map old id → new inner id
+    std::map<int, int> outer_vid;  // vertex ID → new outer vertex ID
+    std::map<int, int> inner_vid;  // vertex ID → new inner vertex ID
 
     for (size_t i = 0; i < body.num_vertices(); ++i) {
-        Point3D p_outer = body.vertex(static_cast<int>(i)).point;
+        const auto& v = body.vertex(static_cast<int>(i));
+        int vid = v.id;
+        Point3D p_outer = v.point;
         Point3D p_inner = offset_vertex(body, static_cast<int>(i), -thickness);
-        outer_vid[i] = result.add_vertex(p_outer);
-        inner_vid[i] = result.add_vertex(p_inner);
+        outer_vid[vid] = result.add_vertex(p_outer);
+        inner_vid[vid] = result.add_vertex(p_inner);
     }
 
     std::vector<int> all_faces;