feat: Add UIDrawable children collection to Grid

Grid now supports a `children` collection for arbitrary UIDrawable elements
(speech bubbles, effects, highlights, path visualization, etc.) that
automatically transform with the grid's camera (pan/zoom).

Key features:
- Children positioned in grid-world pixel coordinates
- Render after entities, before FOV overlay (proper z-ordering)
- Sorted by z_index, culled when outside visible region
- Click detection transforms through grid camera
- Automatically clipped to grid boundaries via RenderTexture

Python API:
  grid.children.append(caption)  # Speech bubble follows grid camera
  grid.children.append(circle)   # Highlight indicator

Closes #132

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

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

src/UIGrid.h

@@ -75,7 +75,11 @@ public:
     sf::RenderTexture renderTexture;
     std::vector<UIGridPoint> points;
     std::shared_ptr<std::list<std::shared_ptr<UIEntity>>> entities;
-    
+
+    // UIDrawable children collection (speech bubbles, effects, overlays, etc.)
+    std::shared_ptr<std::vector<std::shared_ptr<UIDrawable>>> children;
+    bool children_need_sort = true;  // Dirty flag for z_index sorting
+
     // Background rendering
     sf::Color fill_color;
     
@@ -118,6 +122,7 @@ public:
     static PyObject* py_compute_astar_path(PyUIGridObject* self, PyObject* args, PyObject* kwds);
     static PyMethodDef methods[];
     static PyGetSetDef getsetters[];
+    static PyObject* get_entities(PyUIGridObject* self, void* closure);
     static PyObject* get_children(PyUIGridObject* self, void* closure);
     static PyObject* repr(PyUIGridObject* self);