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failed / cancelled 兜底两种末帧路径补「末帧所在屏停留 Xs」, 新增 StuckPoint.dwell_ms(与 stuck_ms 语义分离),dwell-only 显示 规避「总帧数配末段时长」误导;缺 ts/时钟回退降级只显环节。 Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>