feat(scenegraph): add SceneGraphBuilder for pipeline-driven graph construction

Introduce SceneGraphBuilder + SceneGraphBuildConfig to decouple scene graph
construction from the verification notebook. The builder handles batch
inference, hash encoding, and object node creation internally.

- Add SceneGraphBuilder.build_from_room_views() as the main entry point
- Add SceneGraphBuildConfig for inference_batch_size and position strategy
- Add SceneGraphBuildArtifacts to carry cropped images and debug metadata
- Extend ObjectNode with optional detection metadata (label, confidence,
  bbox_xyxy, source_view_id, position_confidence)
- Add RoomView frozen dataclass as a structured view container
- Add flatten_room_views() utility to replace inline list comprehensions
- Refactor notebooks/verification.py to use the new builder API

BREAKING CHANGE: ObjectNode now accepts additional optional fields; direct
scene_graph.objects[obj_id] = ObjectNode(...) construction in the notebook
is replaced by builder.build_from_room_views(...).

tests/test_notebook_verification_static.py

@@ -16,5 +16,18 @@ def test_verification_notebook_uses_scenegraph_query_api():
 def test_verification_notebook_uses_hash_codec_for_object_hashes():
     source = NOTEBOOK.read_text()
 
-    assert "bits_tensor_to_hash_bytes" in source
+    # Notebook should no longer do hash conversion directly;
+    # the builder handles it internally.
+    assert "bits_tensor_to_hash_bytes" not in source
     assert "np.packbits" not in source
+    assert "scene_graph.objects[_obj_id] = ObjectNode" not in source
+
+
+def test_verification_notebook_uses_scene_graph_builder():
+    source = NOTEBOOK.read_text()
+
+    assert "SceneGraphBuilder" in source
+    assert "SceneGraphBuildConfig" in source
+    assert "flatten_room_views" in source
+    assert "scene_graph, build_artifacts = builder.build_from_room_views" in source
+    assert "scene_graph.objects[_obj_id] = ObjectNode" not in source