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

notebooks/verification.py

@@ -38,10 +38,13 @@ def project_imports():
         RoomNode,
         SceneGraphBuildConfig,
         SceneGraphBuilder,
+        load_scene_graph,
         query_image_against_scene_graph,
+        save_scene_graph,
     )
     from simulator import (
         HabitatSimulatorConfig,
+        TopDownRenderStyle,
         TopDownSceneElements,
         collect_room_views_by_room,
         create_habitat_simulator,
@@ -58,16 +61,19 @@ def project_imports():
         RoomNode,
         SceneGraphBuildConfig,
         SceneGraphBuilder,
+        TopDownRenderStyle,
         TopDownSceneElements,
         cfg_manager,
         collect_room_views_by_room,
         create_habitat_simulator,
         flatten_room_views,
+        load_scene_graph,
         numpy_to_pil,
         query_image_against_scene_graph,
         render_topdown_scene_map,
         save_object_image,
         save_room_view,
+        save_scene_graph,
     )
 
 
@@ -194,6 +200,7 @@ def build_scene_graph(
     SceneGraphBuilder,
     cfg_manager,
     habitat_config,
+    load_scene_graph,
     mo,
     numpy_to_pil,
     pipeline,
@@ -201,92 +208,145 @@ def build_scene_graph(
     room_views,
     save_object_image,
     save_room_view,
+    save_scene_graph,
     text_labels,
 ):
     output_dir = cfg_manager.get().output.directory / "verification"
     output_dir.mkdir(parents=True, exist_ok=True)
+    cache_path = output_dir / "scene_graph.json"
 
-    builder = SceneGraphBuilder(
-        pipeline=pipeline,
-        config=SceneGraphBuildConfig(
-            inference_batch_size=4,
-            position_strategy="bbox_depth_center",
-            camera_hfov_degrees=habitat_config.hfov_degrees,
-        ),
-    )
-
-    pil_room_views = [
-        type(_view)(
-            room_id=_view.room_id,
-            view_idx=_view.view_idx,
-            rgb=numpy_to_pil(_view.rgb),
-            depth=_view.depth,
-            agent_position=_view.agent_position,
-            agent_rotation=_view.agent_rotation,
-            camera_position=_view.camera_position,
-            camera_rotation=_view.camera_rotation,
-        )
-        for _view in room_views
-    ]
-
-    with mo.status.spinner(title="Building scene graph from room views"):
-        scene_graph, build_artifacts = builder.build_from_room_views(
-            room_nodes=room_nodes,
-            room_views=pil_room_views,
-            text_labels=text_labels,
+    # Try loading cached scene graph first.
+    if cache_path.exists():
+        scene_graph = load_scene_graph(cache_path)
+        print(f"Loaded cached scene graph from {cache_path}")
+        print(f"  {len(scene_graph.rooms)} rooms, {len(scene_graph.objects)} objects")
+        object_images = {}
+        build_artifacts = None
+    else:
+        builder = SceneGraphBuilder(
+            pipeline=pipeline,
+            config=SceneGraphBuildConfig(
+                inference_batch_size=4,
+                position_strategy="bbox_depth_center",
+                camera_hfov_degrees=habitat_config.hfov_degrees,
+            ),
         )
 
-    object_images = build_artifacts.object_images
-    debug_meta = build_artifacts.debug_meta
+        pil_room_views = [
+            type(_view)(
+                room_id=_view.room_id,
+                view_idx=_view.view_idx,
+                rgb=numpy_to_pil(_view.rgb),
+                depth=_view.depth,
+                agent_position=_view.agent_position,
+                agent_rotation=_view.agent_rotation,
+                camera_position=_view.camera_position,
+                camera_rotation=_view.camera_rotation,
+            )
+            for _view in room_views
+        ]
 
-    # Save original room views.
-    for _room_view in mo.status.progress_bar(
-        pil_room_views,
-        title="Saving room-view snapshots",
-        subtitle="Writing original room images to disk",
-        completion_title="Room-view snapshots saved",
-        completion_subtitle=f"Saved {len(pil_room_views)} room views",
-        show_eta=True,
-        show_rate=True,
-        remove_on_exit=False,
-    ):
-        save_room_view(output_dir, _room_view.room_id, _room_view.view_idx, _room_view.rgb)
+        with mo.status.spinner(title="Building scene graph from room views"):
+            scene_graph, build_artifacts = builder.build_from_room_views(
+                room_nodes=room_nodes,
+                room_views=pil_room_views,
+                text_labels=text_labels,
+            )
 
-    # Save object crops.
-    for _obj_id, _cropped in mo.status.progress_bar(
-        object_images.items(),
-        title="Saving object crops",
-        subtitle="Writing cropped object images to disk",
-        completion_title="Object crops saved",
-        completion_subtitle=f"Saved {len(object_images)} object crops",
-        show_eta=True,
-        show_rate=True,
-        remove_on_exit=False,
-    ):
-        _node = scene_graph.objects[_obj_id]
-        save_object_image(
-            output_dir,
-            _node.room_id,
-            _obj_id,
-            _node.last_seen_frame,
-            0,  # mask idx 0 ok for M0
-            _cropped,
-        )
+        object_images = build_artifacts.object_images
+        debug_meta = build_artifacts.debug_meta
 
-    from collections import Counter
+        # Save scene graph to cache.
+        save_scene_graph(cache_path, scene_graph)
+        print(f"Saved scene graph to {cache_path}")
 
-    _meta_fallbacks = [_meta.get("fallback_reason") for _meta in debug_meta]
-    fallback_count = sum(1 for f in _meta_fallbacks if f is not None)
-    _reasons = Counter(f or "ok" for f in _meta_fallbacks)
-    print(f"Fallback breakdown: {_reasons}")
+        # Save original room views.
+        for _room_view in mo.status.progress_bar(
+            pil_room_views,
+            title="Saving room-view snapshots",
+            subtitle="Writing original room images to disk",
+            completion_title="Room-view snapshots saved",
+            completion_subtitle=f"Saved {len(pil_room_views)} room views",
+            show_eta=True,
+            show_rate=True,
+            remove_on_exit=False,
+        ):
+            save_room_view(output_dir, _room_view.room_id, _room_view.view_idx, _room_view.rgb)
+
+        # Save object crops.
+        for _obj_id, _cropped in mo.status.progress_bar(
+            object_images.items(),
+            title="Saving object crops",
+            subtitle="Writing cropped object images to disk",
+            completion_title="Object crops saved",
+            completion_subtitle=f"Saved {len(object_images)} object crops",
+            show_eta=True,
+            show_rate=True,
+            remove_on_exit=False,
+        ):
+            _node = scene_graph.objects[_obj_id]
+            save_object_image(
+                output_dir,
+                _node.room_id,
+                _obj_id,
+                _node.last_seen_frame,
+                0,  # mask idx 0 ok for M0
+                _cropped,
+            )
+
+        from collections import Counter
+
+        _meta_fallbacks = [_meta.get("fallback_reason") for _meta in debug_meta]
+        fallback_count = sum(1 for f in _meta_fallbacks if f is not None)
+        _reasons = Counter(f or "ok" for f in _meta_fallbacks)
+        print(f"Fallback breakdown: {_reasons}")
+        print(f"Fallback frames: {fallback_count}/{len(debug_meta)}")
 
     print(f"Created {len(scene_graph.objects)} objects")
-    print(f"Saved cropped images to: {output_dir}")
-    print(f"Fallback frames: {fallback_count}/{len(debug_meta)}")
+    print(f"Output directory: {output_dir}")
 
     return build_artifacts, object_images, output_dir, scene_graph
 
 
+@app.cell
+def render_scene_graph_birdseye(
+    TopDownRenderStyle,
+    TopDownSceneElements,
+    meters_per_pixel,
+    mo,
+    render_topdown_scene_map,
+    room_nodes,
+    scene_graph,
+    sim,
+):
+    """Bird's-eye view of the full scene graph: rooms, objects, and edges."""
+    _object_nodes = list(scene_graph.objects.values())
+    _edges = [
+        (_obj.room_id, _obj.obj_id) for _obj in _object_nodes
+    ]
+
+    _style = TopDownRenderStyle(
+        title="Scene Graph — Bird's-Eye View",
+        figure_size=(10, 10),
+    )
+
+    _elements = TopDownSceneElements(
+        room_nodes=room_nodes,
+        object_nodes=_object_nodes,
+        edges=_edges,
+    )
+
+    _birdseye_image = render_topdown_scene_map(
+        pathfinder=sim.pathfinder,
+        elements=_elements,
+        meters_per_pixel=meters_per_pixel,
+        style=_style,
+    )
+
+    mo.md("## Scene Graph Bird's-Eye View"), mo.image(_birdseye_image)
+    return (_birdseye_image,)
+
+
 @app.cell
 def build_tables(pl, scene_graph):
     room_rows = [
@@ -335,6 +395,40 @@ def display_tables(mo, objects_df, rooms_df):
     return
 
 
+@app.cell
+def display_objects_by_room(mo, objects_df, pl):
+    """Table of all objects with their room assignments."""
+    _obj_room_df = objects_df.select(
+        ["obj_id", "room_id", "label", "confidence", "position_confidence"]
+    ).sort(["room_id", "label"])
+
+    mo.vstack([
+        mo.md("## Objects by Room"),
+        mo.ui.table(_obj_room_df),
+    ])
+    return
+
+
+@app.cell
+def display_room_summary(mo, objects_df, pl, rooms_df):
+    """Table of all rooms with their object counts."""
+    _counts = (
+        objects_df.group_by("room_id")
+        .agg(pl.col("obj_id").count().alias("object_count"))
+    )
+    _room_summary = rooms_df.select(["room_id"]).join(
+        _counts, on="room_id", how="left"
+    ).with_columns(
+        pl.col("object_count").fill_null(0).cast(pl.Int64)
+    ).sort("room_id")
+
+    mo.vstack([
+        mo.md("## Room Summary"),
+        mo.ui.table(_room_summary),
+    ])
+    return
+
+
 @app.cell
 def upload_query(mo):
     file_upload = mo.ui.file(