Parallelize the enqueueForUpload collect loop (preparer's internal
bounded semaphore from the previous task keeps IO in check). Also two
test hardenings from that task's code review: pin that failed and
thrown prepare operations release their concurrency slot (cap=1,
subsequent prepare still completes), and pin the maxConcurrentPrepares
<1 -> 1 clamp (peak 1, no deadlock).
Co-Authored-By: Claude Code <noreply@anthropic.com>
浅念
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