feat(editor): gradient texture blend for biome transitions

- Gradient Blend: smoothly transitions base texture from one biome
  to another across the entire tile (horizontal or vertical)
- Preset buttons: Grass→Sand and Grass→Snow gradients
- Creates natural biome transition zones without manual painting
- Alpha blending from 0% to 100% across 16 chunks

tools/editor/texture_painter.hpp

@@ -28,6 +28,9 @@ public:
     void paintAlongPath(const glm::vec3& start, const glm::vec3& end,
                         float width, const std::string& texturePath);
 
+    // Gradient blend: transitions base texture from one to another across tile
+    void gradientBlend(const std::string& tex1, const std::string& tex2, bool horizontal);
+
     // Paint the active texture at the given world position
     // Returns list of modified chunk indices
     std::vector<int> paint(const glm::vec3& center, float radius, float strength, float falloff);