Add dl_source2: dual download source with speed test and fallback

[neyoneit]

Summary

• Adds dl_source2 cvar as a secondary map download source
• On first download per session, runs a 3-second parallel speed test to determine the faster source
• If the primary source fails, automatically falls back to the secondary source
• Auto-fills empty source cvars with known mirrors (ws.q3df.org / defrag.racing)
• Detects and fixes duplicate sources pointing to the same server
• Auto-corrects HTTPS to HTTP for ws.q3df.org (only works over HTTP)

[crabcrabcam]

LLM/AI generated code is of unknown source and incompatible with the GPL this software is under.

[neyoneit]

LLM/AI generated code is of unknown source and incompatible with the GPL this software is under.

I am the maintainer of this repository and the author of this PR. I take full responsibility for the code and its GPL compliance. Using an AI assistant does not transfer authorship.

[wolf6542]

LLM/AI generated code is of unknown source and incompatible with the GPL this software is under.

This is legally not true, because John Doe v. GitHub (2022) is not settled (although most of the plantiff's claims have been dismissed as of right now).
They have been dismissed largely because it is extremely difficult to demonstrate in a court of law the origin and therefore the cause and effect, that's because AI works stochastically. Even if you could demonstrate that a piece of logic is prominent enough and distinct in its origin from an X source that is not GPL compatible, your work is still ahead of you in terms of litigating a claim - copyright protects the expression, not the idea, a “similar idea” doesn't automatically equal infringement.
P.S. it's true, technically you could introduce license contamination with AI, but you could also do that without AI - taking code from other OS projects or even StackOverflow, etc. AI just makes this process a lot easier and largely untraceable which in my humble opinion (as a lawyer) makes it for all intents and purposes extremely difficult to litigate against.