feat(v3.9): parallel OpenMP boolean + assembly instancing + update notifier

include/vde/brep/update_notifier.h

@@ -0,0 +1,41 @@
+#pragma once
+#include "vde/brep/feature_tree.h"
+#include <functional>
+#include <string>
+#include <vector>
+
+namespace vde::brep {
+
+/// Callback type for model changes.
+/// Receives the feature name and the new model after rebuild.
+using UpdateCallback = std::function<void(const std::string& feature_name, const BrepModel& new_model)>;
+
+/**
+ * @brief Notifies listeners when features change.
+ *
+ * Integrates with FeatureHistory: after each apply/undo/redo,
+ * call notify() to push the updated model to all registered callbacks.
+ *
+ * @ingroup brep
+ */
+class UpdateNotifier {
+public:
+    /// Register a callback. Called on every notify().
+    void on_update(UpdateCallback cb) { callbacks_.push_back(std::move(cb)); }
+
+    /// Notify all registered listeners.
+    void notify(const std::string& feature, const BrepModel& model) {
+        for (auto& cb : callbacks_) cb(feature, model);
+    }
+
+    /// Remove all callbacks.
+    void clear() { callbacks_.clear(); }
+
+    /// Number of registered callbacks.
+    [[nodiscard]] size_t listener_count() const { return callbacks_.size(); }
+
+private:
+    std::vector<UpdateCallback> callbacks_;
+};
+
+} // namespace vde::brep