Foreach fan-out stalls when one split OOMs

The brief

A single out-of-memory foreach split stalls the entire fan-out with no signal about which branch died. Dead-branch detection with partial results is needed.

Acceptance criteria

• Dead split surfaces within one polling interval with its branch id
• Healthy branches complete and their results stay retrievable
• No behaviour change for successful fan-outs
• Docs and an integration test with a poisoned split included