Previously addJobs() was fire-and-forget — added jobs ran as orphaned
promises. When the original batch completed, batch-done fired and
uploadManager was set to null while added jobs were still running.
Now: added job promises are tracked in _additionalPromises array.
startBatch drains this array after the original tasks complete,
ensuring batch-done only fires when ALL jobs (original + added) finish.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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