feat(scenegraph): add JSON serialization and implement top-down object/edge rendering

- Add save_scene_graph() and load_scene_graph() for persisting scene graphs to JSON
- Implement object node overlay (colored by label) in render_topdown_scene_map
- Implement edge arrows (room → object) in render_topdown_scene_map
- Add TopDownRenderStyle fields for object/edge visual configuration
- Add scene graph caching to verification notebook to skip rebuilds
- Add render_scene_graph_birdseye cell for full scene graph visualization
- Add display_objects_by_room and display_room_summary cells

mini-nav/scenegraph/__init__.py

@@ -21,6 +21,7 @@ from .query import (
 )
 from .roomnode import RoomNode
 from .scenegraph import SceneGraphMatch, SimpleSceneGraph
+from .serialization import load_scene_graph, save_scene_graph
 from .software_cam import CamMatch, SoftwareCamIndex, xnor_popcount_score
 
 __all__ = [
@@ -40,6 +41,8 @@ __all__ = [
     "cam_row_to_hash_bytes",
     "hash_bytes_to_bits_array",
     "hash_bytes_to_cam_row",
+    "load_scene_graph",
     "query_image_against_scene_graph",
+    "save_scene_graph",
     "xnor_popcount_score",
 ]