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Code Review

This pull request introduces a _StreamMultiplexer to handle concurrent download tasks over a single bidirectional gRPC stream, replacing the previous locking mechanism in AsyncMultiRangeDownloader. The multiplexer routes responses to per-task asyncio queues based on read_id, allowing for better resource utilization. The lock parameter in download_ranges is now deprecated. Feedback focuses on improving the multiplexer's reliability and performance, specifically by using asyncio.gather to prevent head-of-line blocking during response broadcasting, ensuring the background receive loop terminates when no tasks are active, and adding error logging for observability.