feat(editor): height clamp tool for controlled terrain range

- Clamp Heights: sets min/max height bounds across entire tile
  (DragFloatRange2 slider for min/max, -500 to 2000 range)
- Useful workflow: Generate noise → Smooth → Clamp to desired range
- Prevents terrain from going underground or too high
- All affected chunks marked dirty for mesh regeneration

tools/editor/terrain_editor.hpp

@@ -57,6 +57,9 @@ public:
     // Global smooth pass across entire tile (N iterations)
     void smoothEntireTile(int iterations);
 
+    // Clamp all heights to a min/max range
+    void clampHeights(float minH, float maxH);
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);