Flashback Turbo

Audit cycle 1 — dead code cleanup

No behavior change at runtime. Internal cleanup after a static audit:

• Removed 2 dead config fields (nativeAlphaCleanup, gpuColorspaceConversion) that were placeholders for hooks never implemented and had 0 references in the codebase. The fields are silently ignored if still present in your existing config/flashbackturbo.json.
• Added scripts/build-26.1.sh to mechanically derive the 26.1.x build from the 1.21.x source (yarn→mojang remapping). Internal tooling, prevents drift between the two branches.

All H2–H9 hooks unchanged from 0.3.5. If 0.3.5 works for you, this update is purely cosmetic.