Document how a connector handles a device that decides its own ports

An undeclared key fails the whole reading, so a connector whose ports vary by
model has to narrow what it publishes to what was bound. Names the idiom and
the once-not-per-poll rule for saying a declared port is missing.

docs/reference/connector-contract.md

@@ -97,6 +97,27 @@ optional `interval`.
   output port publishes at most every *n* seconds, an input port wakes its node
   at most that often. A connector should poll at the rate the device is
   comfortable with and leave delivery rates to whoever wires it up.
+- A key no port declares is an error, and it fails the whole reading rather
+  than the one value — a mistyped metric name is how a training curve goes
+  missing.
+
+### When the device decides what the ports are
+
+A connector for a device whose readings vary by model — which components a
+relay has, which entities were flashed onto a board — cannot know its ports in
+advance, and returning everything the device reports would fail on the first
+value nobody bound. Narrow the reading to the ports that were declared:
+
+```python
+declared = {spec.port for spec in self.output_ports if spec.name}
+return {port: value for port, value in reading.items() if port in declared}
+```
+
+`spec.port` is the local, unqualified name and stays that way for the node's
+whole life, so the set can be taken fresh each time. Say something in the log
+when a declared port is not one the device has — once, not once per poll: from
+the canvas a renamed entity and a typo look the same, and both leave a port
+silent forever.
 
 ## Polling