feat(v4.3): BOM (bill of materials) — flat merged, name-sorted

- BomEntry struct: name, quantity, volume, total_volume
- generate_bom(): recursive assembly traversal, same-name merging
- 5 new tests, 26/26 passing

src/brep/measure.cpp

@@ -4,7 +4,10 @@
 #include "vde/core/aabb.h"
 #include "vde/mesh/halfedge_mesh.h"
 #include <cmath>
+#include <functional>
 #include <limits>
+#include <map>
+#include <string>
 #include <algorithm>
 
 namespace vde::brep {
@@ -333,4 +336,36 @@ double clearance(const BrepModel& a, const BrepModel& b) {
     return distance(a, b);
 }
 
+// ── generate_bom ────────────────────────────────────────────
+
+std::vector<BomEntry> generate_bom(const AssemblyNode& node) {
+    std::map<std::string, BomEntry> bom_map;
+
+    std::function<void(const AssemblyNode&)> traverse =
+        [&](const AssemblyNode& n) {
+            if (n.is_part() && n.model.has_value()) {
+                double vol = volume(*n.model);
+                auto& entry = bom_map[n.name];
+                if (entry.quantity == 0) {
+                    entry.name = n.name;
+                    entry.volume = vol;
+                }
+                entry.quantity++;
+                entry.total_volume += vol;
+            }
+            for (const auto& child : n.children) {
+                traverse(*child);
+            }
+        };
+
+    traverse(node);
+
+    std::vector<BomEntry> result;
+    result.reserve(bom_map.size());
+    for (auto& [_, entry] : bom_map) result.push_back(std::move(entry));
+    std::sort(result.begin(), result.end(),
+              [](const BomEntry& a, const BomEntry& b) { return a.name < b.name; });
+    return result;
+}
+
 } // namespace vde::brep