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_dashboards.py

@@ -187,3 +187,64 @@ def test_publishing_nothing_does_nothing():
     pipeline.publish({})
 
     assert pipeline.values() == {}
+
+
+def query_chart(**config) -> dict:
+    return {
+        "source": "query",
+        "request": "heating.chart_req",
+        "request_dtype": "record",
+        "message": "heating.chart_series",
+        "dtype": "series",
+        "refresh_s": 30,
+        **config,
+    }
+
+
+def test_a_widget_refuses_a_dtype_it_cannot_carry():
+    WidgetDef(id="s", type="switch", config={"message": "a.b", "dtype": "bool"})
+    # A document written before the editor recorded types binds anything.
+    WidgetDef(id="s", type="switch", config={"message": "a.b"})
+
+    with pytest.raises(ValueError):
+        WidgetDef(id="s", type="switch", config={"message": "a.b", "dtype": "float"})
+
+
+def test_the_structured_widgets_bind_their_shapes():
+    WidgetDef(id="a", type="agenda", config={"message": "a.b", "dtype": "list"})
+    WidgetDef(id="n", type="notification", config={"message": "a.b", "dtype": "record"})
+
+    with pytest.raises(ValueError):
+        WidgetDef(id="a", type="agenda", config={"message": "a.b", "dtype": "json"})
+    with pytest.raises(ValueError):
+        WidgetDef(
+            id="n", type="notification", config={"message": "a.b", "dtype": "list"}
+        )
+
+
+def test_a_querying_chart_asks_with_a_record_and_draws_a_series():
+    widget = WidgetDef(id="c", type="chart", config=query_chart())
+
+    # It publishes its request and reads the answer, so the canvas draws both.
+    assert widget.target == "heating.chart_req"
+    assert widget.messages == ["heating.chart_series"]
+
+    with pytest.raises(ValueError):
+        WidgetDef(id="c", type="chart", config=query_chart(dtype="float"))
+    with pytest.raises(ValueError):
+        WidgetDef(id="c", type="chart", config=query_chart(request_dtype="json"))
+
+
+def test_a_querying_chart_keeps_no_ring():
+    """The answer carries its own past; a ring would store it twice."""
+    widget = WidgetDef(
+        id="c", type="chart", config=query_chart(history={"points": 900})
+    )
+
+    assert widget.history_points == 0
+
+
+def test_a_dashboard_is_cut_into_a_sane_number_of_columns():
+    for columns in (0, 49):
+        with pytest.raises(ValueError):
+            DashboardDef(name="house", columns=columns)