feat: LOD mesh generation + incremental BVH

include/vde/spatial/incremental_bvh.h

@@ -0,0 +1,52 @@
+#pragma once
+#include "vde/spatial/bvh.h"
+#include <functional>
+#include <vector>
+
+namespace vde::spatial {
+
+/**
+ * @brief Incremental BVH that supports insert/remove without full rebuild
+ *
+ * Wraps the standard BVH with a dirty flag. Triangles can be inserted,
+ * removed, or updated incrementally. The tree is lazily rebuilt on the
+ * next query when dirty. Suitable for moderately dynamic scenes where
+ * queries are less frequent than modifications.
+ *
+ * @ingroup spatial
+ */
+class IncrementalBVH {
+public:
+    /// @brief Build from initial set of triangles
+    void build(const std::vector<core::Triangle3D>& triangles);
+
+    /// @brief Insert a new triangle (marks dirty, no immediate rebuild)
+    void insert(const core::Triangle3D& tri);
+
+    /// @brief Remove a triangle by index (marks dirty)
+    void remove(size_t index);
+
+    /// @brief Update a triangle's geometry in-place (marks dirty)
+    void update(size_t index, const core::Triangle3D& tri);
+
+    /**
+     * @brief Ray query (rebuilds if dirty, then delegates to BVH)
+     *
+     * @param ray Query ray
+     * @return All hit results (t, triangle, hit point)
+     */
+    std::vector<BVH::HitResult> ray_query(const Ray3Dd& ray) const;
+
+    /// @brief Number of triangles
+    [[nodiscard]] size_t size() const { return triangles_.size(); }
+
+private:
+    std::vector<core::Triangle3D> triangles_;
+    std::vector<core::AABB3D> bboxes_;
+    BVH bvh_;
+    bool dirty_ = true;
+
+    void rebuild_if_dirty();
+};
+
+} // namespace vde::spatial