[BUG] 在空模型缓存上首次运行:每个摄像头工作进程都同时下载和转换同一个模型,一个工作进程出现故障,后期管线就永远等待不了了(2.0.0-alpha.23)

Hi all, second report from the same RTMPose evaluation (see #886 for the first). We processed a six-camera recording through alpha.23 on a fresh install, and the very first run stalled. Worth a report because anyone processing six or more cameras on a new machine will hit it. ** Describe the bug ** The posthoc pipeline starts one VideoNode per camera at the same time. On a cold cache, each of them downloads and converts the same model files into the shared cache. They race each other, one of them reads a half-written file and dies while building its session, and the aggregator then waits for a worker that will never report. The pipeline stays `alive`, so a headless caller never returns and the UI never shows completion. We hit it on both tracker paths: - RTMPose (ONNX models in `~/.cache/skellytracker/models/`), six workers, one dies: