feat(editor): crater generator for lakes, arenas, impact sites

- Crater Generator in Sculpt panel: creates a bowl-shaped depression
  with configurable radius, depth, and raised rim height
- Parabolic bowl interior with sinusoidal rim and smooth outer falloff
- Perfect for creating lakes (fill with Water mode), arenas, or
  impact craters for volcanic zones
- One click at cursor position, uses brush position for center

tools/editor/terrain_editor.hpp

@@ -81,6 +81,9 @@ public:
     // Flatten a road between two points (smooths to average height along path)
     void flattenRoad(const glm::vec3& start, const glm::vec3& end, float width);
 
+    // Create a crater at a position (bowl shape with raised rim)
+    void createCrater(const glm::vec3& center, float radius, float depth, float rimHeight);
+
     // Import/export heightmap (raw 16-bit grayscale, 129x129)
     bool importHeightmap(const std::string& path, float heightScale);
     bool exportHeightmap(const std::string& path, float heightScale);