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/app/flow/connector.py

@@ -14,7 +14,10 @@ What a connector gets from the base class:
 * :meth:`Node.report_health`, so a connection problem shows on the node rather
   than only in the log;
 * the lifecycle hooks the controller drives, so nothing device-specific has to
-  be known by the engine.
+  be known by the engine;
+* :meth:`ConnectorNode.write`, the other direction — values arriving on the
+  node's input ports, for a connector that commands something rather than only
+  reading it.
 
 The message schemas and the parameter model are the rest of the contract, and
 they are the same ones the built-in nodes use. See ``docs/connectors/`` for the
@@ -81,16 +84,15 @@ class ConnectorNode(Node):
     __slots__ = ("config", "_poll_task", "_stop_event", "_last_published")
 
     def __init__(self, **kwargs: Any) -> None:
-        super().__init__(f=self._unused, **kwargs)
+        super().__init__(f=self._dispatch, **kwargs)
         self.config = type(self).Params(**self.params)
         self._poll_task: asyncio.Task[None] | None = None
         self._stop_event: asyncio.Event | None = None
         self._last_published: dict[str, Any] = {}
 
-    @staticmethod
-    def _unused(**_: Any) -> None:
-        """A connector publishes from its own loop, not from the scheduler."""
-        return None
+    def _dispatch(self, params: dict[str, Any], **ports: Any) -> dict[str, Any] | None:
+        """The scheduler's entry point. Settings are already on ``self.config``."""
+        return self.write(**ports)
 
     # -------------------------------------------------------------------------
     # What a connector implements
@@ -104,6 +106,18 @@ class ConnectorNode(Node):
         """
         return None
 
+    def write(self, **ports: Any) -> dict[str, Any] | None:
+        """Send incoming values to the device. Values arrive keyed by input port.
+
+        A connector that only reads leaves this alone — the default discards
+        whatever reaches it, which is what a node with no inputs gets anyway.
+        Return ``None`` unless the device answers something worth publishing,
+        in which case return it keyed by output port like :meth:`poll` does.
+
+        This runs on the scheduler's thread, so it must not block for long.
+        """
+        return None
+
     # -------------------------------------------------------------------------
     # What the engine drives
     # -------------------------------------------------------------------------

backend/app/flow/nodes/mqtt.py

@@ -379,7 +379,11 @@ class MqttNode(Node):
                 )
                 continue
 
-            payload = json.dumps(value)
+            # A string goes on the wire as it stands. Devices on a shared
+            # broker expect bare values, and the subscriber below already
+            # falls back to the raw text when it is not JSON, so a
+            # fluksio-to-fluksio round trip is unaffected.
+            payload = value if isinstance(value, str) else json.dumps(value)
             await client.publish(
                 topic,
                 payload=payload,