feat(editor): smooth beaches tool for natural water-land transitions

- Smooth Beaches button in Water panel: creates gentle beach slopes
  near water level by blending terrain toward waterline height
- Configurable beach width (15 units default)
- Quadratic falloff creates natural concave beach profile
- Use after filling water to soften harsh land-water edges
- Workflow: sculpt → fill water → smooth beaches → paint sand

tools/editor/terrain_editor.hpp

@@ -117,6 +117,9 @@ public:
     // Fill entire tile with water at a height
     void fillWater(float height, uint16_t liquidType);
 
+    // Smooth terrain near water level to create natural beaches
+    void smoothBeaches(float waterHeight, float beachWidth);
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);