perf(editor): cache M2/WMO models across rebuilds, only clear instances

The editor's rebuildObjects path was destroying every cached model and
re-uploading it on every (debounced) change. Added M2Renderer::clearInstances
that drops only the instance list while keeping models loaded. Editor's
clearObjects switches to clearInstances (M2) + clearInstances (WMO),
and persistent path->modelId maps survive across rebuilds. clearTerrain
fully evicts when loading a new zone.

include/rendering/m2_renderer.hpp

@@ -357,6 +357,9 @@ public:
     void removeInstances(const std::vector<uint32_t>& instanceIds);
     void setSkipCollision(uint32_t instanceId, bool skip);
     void clear();
+    /** Drop all instances but keep models in GPU memory. Cheap path for the
+     *  editor's rebuild loop where the same model is re-instanced repeatedly. */
+    void clearInstances();
     void cleanupUnusedModels();
 
     bool checkCollision(const glm::vec3& from, const glm::vec3& to,