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/workers.md

@@ -100,8 +100,12 @@ you want once it does.
   other runs of the same node alone.
 - **If the worker disappears mid-call**, the run fails in seconds with
   `worker went away mid-call` rather than waiting out its timeout.
-- **A worker sends a heartbeat while it executes**, so a long node is
-  distinguishable from a dead socket. Ninety seconds of silence is gone.
+- **A worker sends a heartbeat every ten seconds while it executes**, so a long
+  node is distinguishable from a dead socket. Ninety seconds of nothing at all —
+  not even a heartbeat — fails the call as gone. A heartbeat says the *agent* is
+  alive and nothing about the node, so it never satisfies a node's own timeout:
+  one set to thirty seconds fires after thirty seconds of the node reporting
+  nothing, wherever it runs.
 
 ## Artifacts across machines