Add fast retransmit of lost packets and packet redundancy

[11EJDE11]

Vanilla recalculates its retransmit timeout as RetryDelta = Response_Time() + 10. The problem is that Response_Time() is measured from ACKs, including ACKs for retransmitted packets. If a packet is lost, its ACK arrives later than normal, increasing Response_Time(). That increases RetryDelta, which delays the next retransmission, causing the timeout to grow even further.

This PR addresses the problem in two ways:

• FastRetransmit maintains a separate per-connection RTT estimate using Karn's algorithm, which ignores RTT samples from retransmitted packets. This keeps the estimate from being skewed by packet loss. It only reduces the global RetryDelta; it never increases it. An optional RetransmitBackoff slightly increases the timeout for repeated retries of the same packet to avoid excessive retransmissions.

• PacketRedundancy sends reliable command packets twice, allowing a single lost packet to be recovered from the duplicate without waiting for a retransmission. AdaptiveRedundancy (enabled by default) only sends duplicates to peers that have recently experienced packet loss, reducing unnecessary bandwidth.

Metric	Baseline	FastRetransmit (no backoff)	FastRetransmit (backoff, default)	PacketRedundancy (no adaptive)	PacketRedundancy (adaptive, default)
Effective sim-FPS	11.6-12.6	43.6-51.2	43.8-44.7	42.0-43.1	45.5-48.6
Stall time	73-76%	11.8-23.5%	21.7-23.4%	24.5-26.3%	15.3-17.2%

Before (no fixes):

2026-08-05.15-52-14.mp4

After:

2026-08-05.15-49-40.mp4

Draft until: Phobos-developers/YRpp#75

[github-actions]

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• spawner-CnCNetYR-a1a5d8198797752b08d02969e088091154bc00d5.zip
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