review: address CarolinePascal feedback

- language timestamps: float64 -> float32 to match LeRobotDataset frame
  timestamps (Arrow struct + HF feature)
- dataset_metadata: hoist `.language` imports to module top — language.py
  has no lerobot imports, so there is no circular-import risk
- dataset_metadata: add a `meta.tools` setter that persists the catalog to
  info.json and reloads `meta.info`
- feature_utils: validate the `language` dtype instead of returning "" —
  warn (non-fatal) when a non-empty value is written at record time
- centralize the scalar-unwrap helper as `lerobot.utils.utils.unwrap_scalar`,
  shared by render_messages_processor and language_render
- docs: move `## Layer 2 — recipe anatomy` ahead of the resolver sections,
  which describe recipe bindings rather than dataset layout
- language_render: note in EMITTED_AT_TOLERANCE_S that persistent rows change
  on a human-action timescale, not the camera frame rate

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/datasets/test_language.py

@@ -45,6 +45,23 @@ def test_language_arrow_schema_has_expected_fields():
     assert isinstance(event_row_type, pa.StructType)
     assert event_row_type.names == ["role", "content", "style", "camera", "tool_calls"]
 
+    # Persistent-row timestamps use float32, matching LeRobotDataset frame timestamps.
+    assert persistent_row_type.field("timestamp").type == pa.float32()
+
+
+def test_validate_feature_language_warns_only_on_non_empty_value(caplog):
+    from lerobot.datasets.feature_utils import validate_feature_language
+
+    # None (the expected record-time value) is silent and non-fatal.
+    with caplog.at_level("WARNING"):
+        assert validate_feature_language("language_persistent", None) == ""
+    assert caplog.records == []
+
+    # A stray non-empty value is dropped later, so we warn rather than fail.
+    with caplog.at_level("WARNING"):
+        assert validate_feature_language("language_persistent", [{"role": "user"}]) == ""
+    assert any("language_persistent" in r.message for r in caplog.records)
+
 
 def test_style_registry_routes_columns():
     assert {"subtask", "plan", "memory", "motion", "task_aug"} == PERSISTENT_STYLES