feat: Add entity.visible_entities() and improve entity.updateVisibility() (closes #113)

Phase 3 of Agent POV Integration:

Entity.updateVisibility() improvements:
- Now uses grid.fov_algorithm and grid.fov_radius instead of hardcoded values
- Updates any ColorLayers bound to this entity via apply_perspective()
- Properly triggers layer FOV recomputation when entity moves

New Entity.visible_entities(fov=None, radius=None) method:
- Returns list of other entities visible from this entity's position
- Optional fov parameter to override grid's FOV algorithm
- Optional radius parameter to override grid's fov_radius
- Useful for AI decision-making and line-of-sight checks

Test coverage in test_perspective_binding.py:
- Tests entity movement with bound layers
- Tests visible_entities with wall occlusion
- Tests radius override limiting visibility

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/UIEntity.h

@@ -89,6 +89,7 @@ public:
     static PyObject* die(PyUIEntityObject* self, PyObject* Py_UNUSED(ignored));
     static PyObject* path_to(PyUIEntityObject* self, PyObject* args, PyObject* kwds);
     static PyObject* update_visibility(PyUIEntityObject* self, PyObject* Py_UNUSED(ignored));
+    static PyObject* visible_entities(PyUIEntityObject* self, PyObject* args, PyObject* kwds);
     static int init(PyUIEntityObject* self, PyObject* args, PyObject* kwds);
 
     static PyObject* get_position(PyUIEntityObject* self, void* closure);