feat(editor): island terrain generator with beach falloff

- Island Generator: creates raised center terrain dropping to edges
  with configurable center height and edge drop depth
- Flat interior plateau → gradual beach slope → steep underwater drop
- Preserves existing noise detail at 30% blend for natural variation
- Add water around the island for instant ocean environment
- Workflow: New Terrain → Island → Noise → Smooth → Auto-paint

tools/editor/terrain_editor.hpp

@@ -93,6 +93,9 @@ public:
     // Create a ridge/mountain range between two points
     void createRidge(const glm::vec3& start, const glm::vec3& end, float width, float height);
 
+    // Create an island shape (raised center, dropping to base at edges)
+    void createIsland(float centerHeight, float edgeDropoff);
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);