feat(robots/so_follower): emit + populate depth keys when use_depth

When an SO follower has a camera configured with use_depth=True (e.g.
a RealSense), the robot now exposes a paired depth feature so the
dataset records both modalities:

- _cameras_ft adds a 2D "<cam>_depth" entry alongside the 3-channel
  color shape; hw_to_dataset_features turns this into
  observation.depth.<cam> with the depth-map marker.
- get_observation reads cam.read_latest_depth() (float32 metric
  meters from the RealSense async depth API) into <cam>_depth so
  build_dataset_frame can route it.

Detection is duck-typed via getattr(..., "use_depth", False) so other
cameras without that attribute keep their RGB-only behaviour unchanged.

Made-with: Cursor

src/lerobot/robots/so_follower/so_follower.py

@@ -68,9 +68,16 @@ class SOFollower(Robot):
 
     @property
     def _cameras_ft(self) -> dict[str, tuple]:
-        return {
-            cam: (self.config.cameras[cam].height, self.config.cameras[cam].width, 3) for cam in self.cameras
-        }
+        features: dict[str, tuple] = {}
+        for cam in self.cameras:
+            cam_cfg = self.config.cameras[cam]
+            features[cam] = (cam_cfg.height, cam_cfg.width, 3)
+            # Cameras with a depth stream (e.g. RealSense with use_depth=True) also
+            # emit a 2D depth feature; hw_to_dataset_features routes 2D shapes to
+            # ``observation.depth.<bare>`` with the depth-map marker.
+            if getattr(cam_cfg, "use_depth", False):
+                features[f"{cam}_depth"] = (cam_cfg.height, cam_cfg.width)
+        return features
 
     @cached_property
     def observation_features(self) -> dict[str, type | tuple]:
@@ -190,6 +197,14 @@ class SOFollower(Robot):
             dt_ms = (time.perf_counter() - start) * 1e3
             logger.debug(f"{self} read {cam_key}: {dt_ms:.1f}ms")
 
+            # Cameras with a depth stream populate a sibling ``<cam>_depth`` key
+            # (consumed by hw_to_dataset_features / build_dataset_frame).
+            if getattr(self.config.cameras[cam_key], "use_depth", False):
+                start = time.perf_counter()
+                obs_dict[f"{cam_key}_depth"] = cam.read_latest_depth()
+                dt_ms = (time.perf_counter() - start) * 1e3
+                logger.debug(f"{self} read {cam_key} depth: {dt_ms:.1f}ms")
+
         return obs_dict
 
     @check_if_not_connected