feat(editor): smooth entire tile, snap-to-ground toggle, object list improvements

- Smooth Entire Tile: global smoothing pass with configurable iterations
  (1-10). Smooths across chunk boundaries for seamless results. Updates
  inner vertices from smoothed outer grid. Great after noise generation.
- Snap to Ground checkbox: on by default, objects placed at terrain
  surface. Disable for floating/airborne objects.
- Random Rotation + Snap Ground checkboxes side-by-side for fast setup
- Toast notifications on noise apply and smooth operations
- Smooth pass uses cross-chunk neighbor averaging for edge continuity

tools/editor/terrain_editor.hpp

@@ -54,6 +54,9 @@ public:
     // Noise generator: applies procedural height noise to the terrain
     void applyNoise(float frequency, float amplitude, int octaves, uint32_t seed);
 
+    // Global smooth pass across entire tile (N iterations)
+    void smoothEntireTile(int iterations);
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);