多代理广播超时时，优先把 sessions_send 改成批次并发+退避重试

• You are operating multi-agent fan-out workflows using sessions_send.
• Gateway logs are available for observing nested lane queue behavior.

1. Measure current fan-out latency and timeout rate for one broadcast run.
2. Replace single-shot N-way fan-out with batched sends (e.g., batch size 2-3).
3. Add per-batch exponential backoff retry for timeout results.
4. Add queue-size and wall-clock guardrails to stop overload cascades early.
5. Track successful deliveries separately from timeout responses for reconciliation.

• Issue reports lane concurrency as hardcoded; config-level nested concurrency control still needs upstream verification（需验证）.
• Timeout may still occur when destination sessions are overloaded; retries must be bounded.

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