feat(editor): hill/valley generator with smooth bell curve shape

- Hill Generator: creates smooth bell-curve hills at cursor position
  with configurable radius and height
- Valley mode: same shape inverted, creates natural depressions
- Uses (1-t^2)^2 falloff for very smooth natural-looking slopes
- Two buttons: "Create Hill" and "Create Valley" for quick terrain
  shaping without switching brush modes

tools/editor/terrain_editor.hpp

@@ -87,6 +87,9 @@ public:
     // Create a mesa/plateau (raised flat area with steep cliff edges)
     void createMesa(const glm::vec3& center, float radius, float height, float edgeSteepness);
 
+    // Create a smooth hill/mountain
+    void createHill(const glm::vec3& center, float radius, float height);
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);