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(...).

mini-nav/scenegraph/objectnode.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from dataclasses import dataclass
 
 import numpy as np
@@ -20,10 +22,20 @@ class ObjectNode:
     hit_count: int = 1  # 被观测到的次数。太低的可以直接过滤掉
     last_seen_frame: int = 0  # 最后一次看到的帧号或时间戳
 
+    # Optional detection metadata (from M0 integration)
+    label: str | None = None
+    confidence: float | None = None
+    bbox_xyxy: tuple[float, float, float, float] | None = None
+    source_view_id: str | None = None
+    position_confidence: float | None = None
+
     def __post_init__(self):
+        self.position = np.asarray(self.position, dtype=np.float32)
         if len(self.visual_hash) != 64:
             raise ValueError("visual_hash must be exactly 64 bytes (512 bits)")
         if len(self.semantic_hash) != 64:
             raise ValueError("semantic_hash must be exactly 64 bytes (512 bits)")
         if self.position.shape != (3,):
             raise ValueError("position must have shape (3,)")
+        if self.bbox_xyxy is not None and len(self.bbox_xyxy) != 4:
+            raise ValueError("bbox_xyxy must have exactly 4 elements")