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/code/nodes.md

@@ -131,13 +131,16 @@ acknowledged from the canvas.
 
 ## Timeouts
 
-`timeout` on a node is how many seconds its code may run before it is stopped.
-The default is 30, and it covers the *first* call's imports, which can be much
-slower than the body — a node importing torch is not being slow, it is loading.
+`timeout` on a node is how many seconds its code may be *silent* before it is
+stopped. A yield or an `emit` resets the clock, and the first call's imports are
+not charged to it — a node importing torch is not being slow, it is loading.
 
-Above 60 seconds, a live flow may deliver the same work again while the node is
-still running. In a batch run, which never redelivers, it is an idle timeout
-instead: silence this long is a kill.
+There is no timeout by default. Training runs for hours and a node that reports
+nothing is usually working, so the engine waits: what fails a call is the worker
+dying, which arrives at once rather than after a deadline. Set a timeout on the
+nodes where silence means stuck — an HTTP call that should answer in seconds,
+a loop that can spin — either per node or, for all of them,
+with `FLOW_NODE_TIMEOUT`. `timeout = 0` says explicitly that this node has none.
 
 ## Running a node somewhere else