Check a generator's yield at the yield that produced it

The worker held each yield one behind, because the last one is the node's
result when the generator returns nothing of its own. Only the engine knows
what ports a node declared, so the check happened when the *next* yield
arrived — a pass late, which for a training loop is however long one epoch
takes.

The worker now sends every yield as it happens and returns whatever its
generator returned; EmitSink holds the last one back and decides at the end
of the call what it was. Old "emit" frames are still handled, so a remote
agent that has not been restarted keeps working.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/fluksio/flow/nodes/base.py

@@ -333,12 +333,19 @@ class Node:
         return self._to_messages(result)
 
     def _drain(self, generator: Iterator[Any]) -> Any:
-        """Publish each yield as it happens; the end of it is the result."""
+        """Publish each yield as it happens; the end of it is the result.
+
+        The last yield is the result when the generator returns nothing of its
+        own, and which one is last is only known once it ends — so a value is
+        published one behind but *checked* the moment it arrives, which is what
+        fails a mistyped port at the yield that produced it.
+        """
         pending: Any = None
         have_pending = False
         try:
             while True:
                 value = next(generator)
+                self._to_messages(value)
                 if have_pending:
                     self.emit(pending)
                 pending, have_pending = value, True