feat(editor): height offset tool for shifting entire terrain up/down

- Offset Heights: shifts all terrain heights by a constant amount
  (-100 to +100 range with Apply button)
- Useful for raising terrain above water level or sinking below
- Slider + Apply button in the Noise Generator section

tools/editor/terrain_editor.hpp

@@ -108,6 +108,9 @@ public:
     // Invert terrain (flip heights around midpoint)
     void invertHeights();
 
+    // Offset all heights by a constant
+    void offsetHeights(float amount);
+
     // Fill entire tile with water at a height
     void fillWater(float height, uint16_t liquidType);