feat: Grid camera defaults to tile (0,0) at top-left + center_camera() method (#169)

Changes:
- Default Grid center now positions tile (0,0) at widget's top-left corner
- Added center_camera() method to center grid's middle tile at view center
- Added center_camera((tile_x, tile_y)) to position tile at top-left of widget
- Uses NaN as sentinel to detect if user provided center values in kwargs
- Animation-compatible: center_camera() just sets center property, no special state

Behavior:
- center_camera() → grid's center tile at view center
- center_camera((0, 0)) → tile (0,0) at top-left corner
- center_camera((5, 10)) → tile (5,10) at top-left corner

Before: Grid(size=(320,240)) showed 3/4 of content off-screen (center=0,0)
After: Grid(size=(320,240)) shows tile (0,0) at top-left (center=160,120)

Closes #169

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/UIGrid.h

@@ -170,6 +170,12 @@ public:
     static PyObject* py_get_dijkstra_path(PyUIGridObject* self, PyObject* args);
     static PyObject* py_compute_astar_path(PyUIGridObject* self, PyObject* args, PyObject* kwds);
     static PyObject* py_entities_in_radius(PyUIGridObject* self, PyObject* args, PyObject* kwds);  // #115
+    static PyObject* py_center_camera(PyUIGridObject* self, PyObject* args);  // #169
+
+    // #169 - Camera positioning
+    void center_camera();  // Center on grid's middle tile
+    void center_camera(float tile_x, float tile_y);  // Center on specific tile
+
     static PyMethodDef methods[];
     static PyGetSetDef getsetters[];
     static PyObject* get_entities(PyUIGridObject* self, void* closure);