feat(editor): reset-to-flat button for terrain, consolidation

- "Reset to Flat" button in Noise Generator section: zeroes all heights
  across entire tile for starting over without creating a new terrain
- Useful workflow: reset → noise → smooth → scale → clamp → auto-paint

tools/editor/terrain_editor.hpp

@@ -60,6 +60,9 @@ public:
     // Clamp all heights to a min/max range
     void clampHeights(float minH, float maxH);
 
+    // Reset all heights to zero (flat terrain)
+    void resetToFlat();
+
     // Scale all heights by a factor (useful for exaggerating or flattening)
     void scaleHeights(float factor);