flow: structured dtypes, and the widgets that read them

A series, record or list message declares its shape instead of riding
DType.JSON, so a widget binds a shape rather than some JSON and a wrong
binding is refused before anything runs. A list declares its item type,
which is what keeps list[float] expressible for a pipeline.

On top of that: an agenda over a list, a notification over a record, and
a dashboard alert channel that publishes engine faults as one — so a
panel can show what went wrong without a flow wiring it by hand.

Also: only None means a node published nothing, a falsy value of the
wrong shape is now the named error it always should have been; and the
gauge's readout says its size is viewBox geometry rather than type scale.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/tests/flow/test_messages.py

@@ -36,6 +36,67 @@ def test_json_dtype_round_trips():
     assert json.loads(json.dumps(value)) == value
 
 
+def test_record_takes_flat_scalars_only():
+    spec = MessageSpec(name="notice", dtype=DType.RECORD)
+    spec.check({"title": "Boiler", "count": 3, "hot": True, "detail": None})
+    for wrong in ({"a": {"nested": 1}}, {"a": [1]}, [], "x"):
+        with pytest.raises(TypeError):
+            spec.check(wrong)
+
+
+def test_series_carries_lines_and_whatever_else_the_answer_echoes():
+    spec = MessageSpec(name="temps", dtype=DType.SERIES)
+    spec.check(
+        {
+            "range_s": 3600,
+            "interval_s": 60,
+            "lines": [{"label": "living", "points": [[1.0, 21.5], [2.0, 21.6]]}],
+        }
+    )
+    spec.check({"lines": []})
+    for wrong in (
+        {"lines": [{"label": "a", "points": [[1.0, 2.0, 3.0]]}]},
+        {"lines": [{"label": "a", "points": [["1", 2.0]]}]},
+        {"lines": [{"label": 1, "points": []}]},
+        {"lines": {}},
+        {},
+    ):
+        with pytest.raises(TypeError):
+            spec.check(wrong)
+
+
+def test_list_items_follow_the_declared_shape():
+    records = MessageSpec(name="agenda", dtype=DType.LIST)
+    records.check([{"title": "Dentist", "ts": 1.0}])
+    records.check([])
+    with pytest.raises(TypeError):
+        records.check([1.0])
+
+    numbers = MessageSpec(name="window", dtype=DType.LIST, item=DType.FLOAT)
+    numbers.check([21.4, 2])
+    for wrong in ([True], ["1"], 1.0):
+        with pytest.raises(TypeError):
+            numbers.check(wrong)
+
+
+def test_the_failing_item_is_named():
+    spec = MessageSpec(name="window", dtype=DType.LIST, item=DType.FLOAT)
+    with pytest.raises(TypeError, match="index 1"):
+        spec.check([1.0, "x"])
+
+
+def test_a_list_holds_one_declared_level():
+    for item in (DType.LIST, DType.SERIES):
+        with pytest.raises(ValueError):
+            MessageSpec(name="x", dtype=DType.LIST, item=item)
+
+
+def test_coerce_parses_structured_text():
+    spec = MessageSpec(name="notice", dtype=DType.RECORD)
+    assert spec.coerce('{"title": "Boiler"}') == {"title": "Boiler"}
+    assert spec.coerce({"title": "Boiler"}) == {"title": "Boiler"}
+
+
 def test_coerce_from_text():
     assert MessageSpec(name="a", dtype=DType.FLOAT).coerce("2.5") == 2.5
     assert MessageSpec(name="a", dtype=DType.INT).coerce("7") == 7