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.

ROADMAP.md

@@ -73,9 +73,15 @@ external interfaces. See `docs/architecture/structure.canvas` → *Backend – M
       `connector-skeleton/`. The registry follows later
 - [x] First real connectors written against that contract from outside the engine:
       WF-RAC aircon, calendar, UniFi presence and Art-Net, in `connectors/`.
-      Built by `make connectors` and installed into the image. Reading only for
-      now — the aircon package cannot produce a command and Art-Net keeps its
-      packets off the wire until `transmit` is switched on
+      Built by `make connectors` and installed into the image. Three of the four
+      read only; the aircon package still cannot produce a command
+- [x] The other direction of the contract: `ConnectorNode.write` receives the
+      node's input ports, so a connector can command something rather than only
+      read it. Additive, so `CONTRACT_VERSION` stays at 1 — before this the base
+      class discarded every message reaching a connector, which made `artnet`'s
+      packet builder unreachable. Art-Net now sends: a per-port `channels` map
+      puts each input on its own DMX channel, `transmit` still gates the socket,
+      and one node owns one universe because a frame carries all 512 levels
 - [x] Node lifecycle as a protocol (`start`/`stop`/`report_health` on `Node`),
       replacing the controller's per-type isinstance chains — the same hooks a
       connector implements, validated on the built-in nodes first