Surface a failing connector poll as node health and a flow issue

The poll loop remembered what it read rather than what it published, so a
value the node could not publish counted as said: the next poll skipped it,
succeeded, and health went back to ok with the port still dark. Remember it
only after inject returns, and report ok last.

A node reporting itself down is now derived into its flow's issues on read
and counted on the health summary, so the canvas marks it and Home says so.
Being down does not stop the flow, and the issue clears by itself when the
node reports well again. The repeating poll warning is logged once per
outage rather than once per tick.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K1moruzue2kTJd3uVisgNk

docs/reference/connector-contract.md

@@ -131,7 +131,9 @@ async def poll(self) -> dict[str, Any] | None:
 - Return `None` when there is nothing new.
 - **Only changed values are published.** A device polled every few seconds
   usually says the same thing, and every publication wakes everything
-  downstream, so the loop compares against what it last published.
+  downstream, so the loop compares against what it last published — what it
+  actually published, so a publication that failed is retried next tick rather
+  than counting as said.
 - Raising is not fatal: it is reported as a health problem and retried on the
   next tick.
 - The loop calls `inject`, which runs the graph, on a worker thread. `poll()`
@@ -213,9 +215,10 @@ self.report_health("degraded", "3 of 5 registers timed out")
 self.report_health("down", str(exc))
 ```
 
-Three values, `ok`, `degraded` and `down`, plus an optional detail string. The
-engine forwards changes to the editor, which shows them on the node. Reporting
-the same status twice is free — only changes are published. The polling loop
+Three values, `ok`, `degraded` and `down`, plus an optional detail string.
+Reporting the same status twice is free — only changes are published. A node
+reporting `down` is named among its flow's issues and counted on the health
+summary on Home; `degraded` means still working, and is not. The polling loop
 already reports around `poll()`; a connector managing its own connection should
 report when it connects and when it loses the connection.