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_senders.py

@@ -64,3 +64,28 @@ def test_a_full_publish_queue_drops_the_oldest():
         assert health == [("degraded", "publish queue full")]
 
     asyncio.run(scenario())
+
+
+def test_a_string_goes_on_the_wire_bare():
+    """Devices on a shared broker expect `ON`, not `"ON"`."""
+
+    class Recorder:
+        def __init__(self) -> None:
+            self.published: list[tuple[str, str]] = []
+
+        async def publish(self, topic, payload, **_):
+            self.published.append((topic, payload))
+
+    node = MqttNode(
+        requires=[
+            MessageSpec(name="plug", port="plug", dtype=DType.STR),
+            MessageSpec(name="level", port="level", dtype=DType.INT),
+        ],
+        params={"topic": {"plug": "actor/plug", "level": "light/level"}},
+    )
+    node.assign_flow("house", "out")
+    client = Recorder()
+
+    asyncio.run(node._publish_with(client, {"plug": "ON", "level": 60}))
+
+    assert client.published == [("actor/plug", "ON"), ("light/level", "60")]