feat(editor): edge ramp tool for seamless multi-tile connections

- Edge Ramp: smoothly transitions tile borders to a target height
  so adjacent tiles can connect seamlessly
- Configurable target height and ramp width (how far in from the edge)
- Quadratic blend for smooth start from edge → interior
- Essential for multi-tile zone creation: ramp each tile's edges to
  match its neighbor's border height

tools/editor/terrain_editor.hpp

@@ -120,6 +120,9 @@ public:
     // Smooth terrain near water level to create natural beaches
     void smoothBeaches(float waterHeight, float beachWidth);
 
+    // Ramp tile edges to a target height for seamless multi-tile joins
+    void rampEdges(float targetHeight, float rampWidth);
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);