Let a connector write, and publish strings bare

Two things stopped the engine commanding this house. ConnectorNode hardwired
its node function to a no-op, so an input message reaching a connector was
discarded and Art-Net's packet builder was unreachable; write() now carries
the input ports, which is additive so the contract version holds. And the MQTT
publisher JSON-encoded every payload, so "ON" went on the wire quoted and the
devices on a shared broker, which speak bare values, ignored it.

seed_house_control.py is the rig: a flow that drives the washing machine plug,
a dimmer and a colour fixture over MQTT, carries the same two as DMX on an
Art-Net node with transmit still off, and a dashboard to drive it by hand.

backend/tests/flow/test_connector.py

@@ -100,6 +100,34 @@ def test_a_failing_poll_reports_down_and_keeps_going():
     assert health[-1][0] == "ok"
 
 
+class Actuator(ConnectorNode):
+    """A connector that commands something instead of reading it."""
+
+    contract = CONTRACT_VERSION
+    title = "Test actuator"
+
+    def __init__(self, **kwargs: Any) -> None:
+        super().__init__(**kwargs)
+        self.commands: list[dict[str, Any]] = []
+
+    def write(self, **ports: Any) -> None:
+        self.commands.append(ports)
+        return None
+
+
+def test_an_incoming_message_reaches_a_connector_that_writes():
+    node = Actuator(requires=[MessageSpec(name="level", dtype=DType.INT)])
+    node.assign_flow("demo", "actuator")
+
+    assert node.execute({"demo.level": 255}) is None
+    assert node.commands == [{"level": 255}]
+
+
+def test_a_read_only_connector_ignores_what_reaches_it():
+    node = a_sensor([])
+    assert node.execute({}) is None
+
+
 def test_a_credential_param_is_marked_for_the_editor():
     schema = Sensor.Params.model_json_schema()
     assert schema["properties"]["poll_interval"]["default"] == 0