feat(editor): terrain mirror X/Y for symmetric zone design

- Mirror X: copies left half of terrain to right half (mirrored)
- Mirror Y: copies top half to bottom half (mirrored)
- Useful for creating symmetric zones, arenas, or balanced landscapes
- Auto-stitches all chunk edges after mirror for seamless results
- UI buttons in Sculpt panel under "Mirror Terrain" section

tools/editor/terrain_editor.hpp

@@ -68,6 +68,10 @@ public:
     void pasteStamp(const glm::vec3& center);
     bool hasStamp() const { return !stampData_.empty(); }
 
+    // Mirror terrain along X or Y axis through tile center
+    void mirrorX();
+    void mirrorY();
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);