feat(editor): terrain terrace/step generator for layered landscapes

- Terrace tool: quantizes terrain heights into N flat shelves
  (like rice paddies, cliff shelves, or stepped pyramids)
- Configurable step count (2-20)
- Finds actual height range and divides evenly
- Auto-stitches chunk edges after terracing
- Useful for creating tiered arenas, agricultural zones, or
  stylized Meso-American terrain

tools/editor/terrain_editor.hpp

@@ -99,6 +99,9 @@ public:
     // Create a winding canyon across the tile
     void createCanyon(float width, float depth, uint32_t seed);
 
+    // Terrace/quantize heights into N steps
+    void terraceHeights(int steps);
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);