refactor: remove UIEntity collision_pos field

- Remove redundant collision_pos field from UIEntity
- Update position getters/setters to use integer-cast position when needed
- Remove all collision_pos synchronization code
- Simplify entity position handling to use single float position field
- Add comprehensive test coverage proving functionality is preserved

This removes technical debt and simplifies the codebase without changing API behavior.

src/UIEntity.h

@@ -40,7 +40,6 @@ public:
     std::vector<UIGridPointState> gridstate;
     UISprite sprite;
     sf::Vector2f position; //(x,y) in grid coordinates; float for animation
-    sf::Vector2i collision_pos; //(x, y) in grid coordinates: int for collision
     //void render(sf::Vector2f); //override final;
 
     UIEntity();