Added Meshbuilder + new RMesh type

src/MeshBuilder.h

@@ -0,0 +1,22 @@
+#pragma once
+
+#include "Common.h"
+#include "ModelLoader.h"
+
+// Lightweight mesh editing state used by the MeshBuilder panel.
+class MeshBuilder {
+public:
+    bool hasMesh = false;
+    RawMeshAsset mesh;
+    std::string loadedPath;
+    bool dirty = false;
+    int selectedVertex = -1;
+
+    bool load(const std::string& path, std::string& error);
+    bool save(const std::string& path, std::string& error);
+    void clear();
+    void recomputeNormals();
+
+    // Add a new face defined by vertex indices (3 = triangle, 4 = quad fan).
+    bool addFace(const std::vector<uint32_t>& indices, std::string& error);
+};