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/concepts/flows.md

@@ -163,10 +163,13 @@ to:
 | `self_loop_needs_initial` | a node reads a message it also writes, with no starting value |
 | `node_error` | the node's code did not load: a syntax error, a missing import |
 | `unauthenticated_hook` | advisory — a webhook with no shared secret is open to anyone |
+| `node_unhealthy` | the node loaded but is not working: a connector that cannot reach its device, or whose last publication failed |
 
-A flow with any of these except the advisory one does not run. The health
-summary on Home counts them, so "why is nothing happening?" has an answer that
-does not involve reading logs.
+A flow with any of these except the advisory one and `node_unhealthy` does not
+run — a node reporting itself down is a live condition, not a build error, so
+the rest of the flow keeps going and the issue clears by itself once the node
+reports well again. The health summary on Home counts them, so "why is nothing
+happening?" has an answer that does not involve reading logs.
 
 ## What happens at runtime