Refuse what a node cannot publish, and stop timing out work that is fine

Four things the python SDK turned up, each fixed where every client sees it.

A key no port declares is now an error rather than a silent drop, on the
return, the yield and the emit alike — the contract the docs already stated.
The SDK reads literal yields at sync time, so a typo fails before anything
runs, and an emission of one fails the call rather than being logged where
nobody looks.

NaN and infinity are refused at the port. JSON cannot spell either, so one
that travelled came back as a 500, a socket frame that stopped the canvas, or
a metric batch the database dropped whole.

An artifact input takes `@run:<id>.<output>` or a bare digest, resolved on the
engine — so the CLI, the run dialog and a python caller mean the same thing,
and a sweep can pass one at all.

Node timeouts are off by default. The clock measured silence, which a training
node is full of, and remote workers had already stopped enforcing it — their
heartbeat reset it. Now a heartbeat proves the agent rather than the node,
ninety seconds of nothing fails the call either way, and the engine touches
work it is still running so a long node is not redelivered at sixty seconds.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019V5bsYGNxcgPs4xXmTPx69

docs/interface/flow-editor.md

@@ -56,10 +56,11 @@ A Function node gets a second settings section where you add your own: a
 setting is a constant of this node's code and arrives as an argument beside its
 ports. See [Where a node's values come from](../concepts/values.md).
 
-**Timeout** (Function nodes) is how many seconds the code may run before it is
-stopped. Once a node streams, this becomes an *idle* timeout: it measures
-silence, not duration, so a node that yields every few seconds can run for
-hours under a timeout of 300.
+**Timeout** (Function nodes) is how many seconds the code may be *silent*
+before it is stopped: it measures silence, not duration, so a node that yields
+every few seconds can run for hours under a timeout of 300. Left empty there is
+no limit, which is the default — set one where a node going quiet means it is
+stuck rather than working. A `0` says so explicitly.
 
 **Code** is the editor. It saves as you type and applies on ⌘S; the node
 reloads without the flow stopping. A node file defines `process(...)`; if it