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_alerts.py

@@ -197,3 +197,36 @@ def test_alerting_can_be_switched_off():
 def test_every_alerting_event_reads_as_a_sentence(event, expected):
     alert = describe(event)
     assert (alert.title if alert else None) == expected
+
+
+def test_a_dashboard_channel_publishes_the_alert_as_a_record():
+    published: list[tuple[str, dict]] = []
+    config = AlertsConfig(
+        channels=[
+            Channel(name="panel", kind="dashboard", config={"message": "house.notice"})
+        ],
+        rules=[Rule(events=[], channels=["panel"])],
+    )
+    alerts = AlertManager(EventBus(), config=config, now=Clock())
+    alerts.publish = lambda name, value: published.append((name, value))
+
+    asyncio.run(alerts.handle(error_event()))
+
+    assert len(published) == 1
+    name, record = published[0]
+    assert name == "house.notice"
+    # Flat named scalars, which is what a notification widget binds.
+    assert record["title"] == "heating.pump failed"
+    assert record["severity"] == "error"
+    assert all(isinstance(v, str) for v in record.values())
+
+
+def test_a_dashboard_channel_without_a_message_says_so():
+    alerts = AlertManager(EventBus(), now=Clock())
+    alerts.publish = lambda name, value: None
+    channel = Channel(name="panel", kind="dashboard")
+
+    with pytest.raises(ValueError):
+        asyncio.run(
+            alerts.send(channel, Alert(title="t", body="b"), raise_on_error=True)
+        )