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(