feat(depth): persist depth metadata + add reader helpers

tests/datasets/test_dataset_metadata.py

@@ -142,6 +142,36 @@ def test_create_without_videos_has_no_video_path(tmp_path):
     assert meta.video_keys == []
 
 
+def test_depth_keys_property_filters_by_marker(tmp_path):
+    """``depth_keys`` selects only video features carrying ``video.is_depth_map=True``."""
+    features = {
+        **SIMPLE_FEATURES,
+        "observation.images.cam": {
+            "dtype": "video",
+            "shape": (64, 96, 3),
+            "names": ["height", "width", "channels"],
+            "info": None,
+        },
+        "observation.depth.cam": {
+            "dtype": "video",
+            "shape": (64, 96),
+            "names": ["height", "width"],
+            "info": {"video.is_depth_map": True},
+        },
+    }
+    meta = LeRobotDatasetMetadata.create(
+        repo_id="test/depth_keys", fps=DEFAULT_FPS, features=features, root=tmp_path / "depth_keys"
+    )
+
+    assert set(meta.video_keys) == {"observation.images.cam", "observation.depth.cam"}
+    assert meta.depth_keys == ["observation.depth.cam"]
+    
+def test_depth_keys_empty_when_no_marker(tmp_path):
+    meta = LeRobotDatasetMetadata.create(
+        repo_id="test/no_depth", fps=DEFAULT_FPS, features=VIDEO_FEATURES, root=tmp_path / "no_depth"
+    )
+    assert meta.depth_keys == []
+
 def test_create_raises_on_existing_directory(tmp_path):
     """create() raises if root directory already exists."""
     root = tmp_path / "existing"