review: emitted_at uses 0.1s tolerance; MessageTurn requires stream at construction

* **Float tolerance in `emitted_at` for persistent styles.** The
  ``_timestamp(row) == t`` exact-equality check silently missed any
  caller that derived ``t`` arithmetically (e.g. ``frame_idx / fps``)
  even though the parquet timestamp would only differ by ULPs. Added
  ``EMITTED_AT_TOLERANCE_S = 0.1`` and check ``abs(...) <= tolerance``
  instead, with a docstring explaining why exact equality wasn't
  enough and why 0.1 s is safe at typical 30–100 Hz control rates.
  Test asserts the new behavior at half-window (matches) and
  double-window (no match) using the constant so it stays in sync.

* **`MessageTurn.stream` is required at construction.** It was typed
  ``MessageStream | None = None`` so YAML could omit ``stream:`` and
  pass the dataclass invariant — but ``_validate_rendered`` rejected
  ``None`` streams later, surfacing the error at the first sample
  instead of at recipe load. Now ``__post_init__`` raises
  ``ValueError`` if ``stream`` is ``None``, with the list of valid
  streams in the message. The redundant late-stage check in
  ``_validate_rendered`` is replaced with a one-line comment that
  cites the upstream invariant. Test pins the new construction-time
  rejection.

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

tests/configs/test_recipe.py

@@ -29,6 +29,17 @@ def test_message_recipe_validates_unknown_binding():
         )
 
 
+def test_message_turn_requires_a_stream():
+    """Every turn must declare a stream — None is rejected at construction.
+
+    Previously this only failed at render time (``_validate_rendered``);
+    catching it here means a malformed recipe YAML errors at load instead
+    of at the first training sample.
+    """
+    with pytest.raises(ValueError, match="missing a stream"):
+        MessageTurn(role="user", content="${task}")
+
+
 def test_message_recipe_requires_at_least_one_target():
     with pytest.raises(ValueError, match="target"):
         TrainingRecipe(

tests/datasets/test_language_render.py

@@ -6,6 +6,7 @@ pytest.importorskip("datasets", reason="datasets is required (install lerobot[da
 
 from lerobot.configs.recipe import MessageTurn, TrainingRecipe  # noqa: E402
 from lerobot.datasets.language_render import (  # noqa: E402
+    EMITTED_AT_TOLERANCE_S,
     active_at,
     emitted_at,
     nth_next,
@@ -342,6 +343,21 @@ def test_resolve_task_explicit_override_beats_rephrasings():
     assert rendered["messages"][0]["content"] == "explicit override wins"
 
 
+def test_emitted_at_persistent_tolerates_small_timestamp_drift():
+    """Persistent ``emitted_at`` should match within EMITTED_AT_TOLERANCE_S
+    so callers that derive ``t`` arithmetically (``frame_idx / fps``) still
+    line up with the parquet-stored timestamp.
+    """
+    rows = [persistent_row("assistant", "memo", "memory", 1.0)]
+    # Half a tolerance window — bit-different float, comfortably inside
+    inside = emitted_at(1.0 + EMITTED_AT_TOLERANCE_S / 2, persistent=rows, events=[], style="memory")
+    assert inside is not None and inside["content"] == "memo"
+
+    # Just past the window — no match
+    outside = emitted_at(1.0 + EMITTED_AT_TOLERANCE_S * 2, persistent=rows, events=[], style="memory")
+    assert outside is None
+
+
 def test_render_sample_rejects_non_dict_language_rows():
     """``_normalize_rows`` must surface malformed inputs as TypeError.