Port to 26.1.2 (Java 25, Mojmap): rebase #4990 on 1.21.11, all loaders incl. Forge

[AndreNijman]

Continues and completes #4990 by @fnltochka (their commits are preserved here). That PR was stuck at CONFLICTING with Forge unfinished; this rebases it onto the current 1.21.11 tip and finishes the loaders.

What changed vs #4990

• Merged latest 1.21.11 and resolved the conflicts. They were all in build config (every Java source auto-merged). The base's recent "update unimined & gradle" commit had already converged on the same Unimined 1.4.2-SNAPSHOT the port needs, which is what unblocked Forge.
  • Kept the 26.1.2 loader versions (forge 64.0.8 / neoforge 73 / fabric 0.19.3).
  • Dropped parchment(...) — there is no 26.1.2 Parchment build — keeping mojmap + devFallbackNamespace "official".
  • Kept the base's newer runs config API (mainClass = ... / args.addAll([...])) and gradle-wrapper hardening.

Validation (built locally on Zulu JDK 25 w/ jmods)

Check	Result
Fabric build	✅ incl. ProGuard (confirms the jmods/JDK-25 path)
NeoForge build	✅
Forge build	✅ — this was the open TODO in #4990
:test	✅ passed
Mixin apply-time check	✅ MixinWorldRenderer#onStartHand params verified against the real 26.1.2 LevelRenderer#renderLevel bytecode (javap)

So this produces standalone jars for all three loaders (Fabric/NeoForge/Forge).

Still wants a human pass

• An in-game smoke test (load a jar, confirm pathfinding) — couldn't run a real client in my build environment. Compile + tests + bytecode-verified mixins give high confidence, but a runtime check is worth doing before release.

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