feat: yamux receive window auto-tuning

[asabya]

Summary

The yamux stream muxer in py-libp2p currently uses a fixed 256KB receive window, which limits throughput on high-bandwidth links. Other libp2p implementations (notably go-yamux) implement receive window auto-tuning that dynamically grows the window up to 16MB based on RTT measurements.

Proposed Change

Implement yamux receive window auto-tuning that:

• Starts at the default 256KB window size
• Doubles the target receive window each RTT epoch when the buffer is being consumed quickly
• Caps at 16MB max receive window (matching go-yamux)
• Sends WINDOW_UPDATE frames to the peer as the window grows

This is the same algorithm used in go-yamux.

Implementation

A working implementation is available in #1269 and is currently being tested in the unified-testing perf suite (see libp2p/unified-testing#62).

Key changes:

• Added MAX_WINDOW_SIZE = 16 * 1024 * 1024 constant
• Added target_recv_window tracking per stream
• Added _auto_tune_and_send_window_update() method that grows the window based on consumption rate
• Called on every read() and _read_stream_data() to keep the window responsive

Impact

Perf test results show significant throughput improvement with auto-tuning enabled, particularly for upload/download benchmarks over tcp+yamux.

[asabya]

libp2p/unified-testing#62 (comment)