为每段助手输出增加 streamId 与 iteration 元数据；前端仅在同一 streamId 内合并流式更新，在 tool_calls_detected 与主通道轮次切换时清流式状态，避免 PE「执行输出」跨轮或工具调用后被覆盖；落库 planning 也按 streamId/iteration 分段，刷新后与线上一致。
Each assistant segment now carries streamId and iteration. The UI only merges chunks within the same stream, clears stream state on tool detection and main-channel round changes, so Plan-Execute “Execution output” no longer overwrites earlier boxes. Persisted planning rows are split by streamId/iteration for consistency after refresh.