Build Sunshine with CUDA support on x86_64

[jeremydixon22]

Summary

• enable Sunshine CUDA support by default on x86_64
• add the CUDA toolkit as an x86_64-only build dependency
• keep aarch64 builds CUDA-free and otherwise unchanged
• preserve the correction as an Omarchy overlay across future AUR syncs

This fixes the packaging path behind omacom/omarchy#7752: generic builders previously had no CUDA package installed, so the upstream detect default disabled the CUDA translation unit and NVIDIA users fell back to software encoding.

Verification

• exact AUR commit 3c38fe4064f8c8a49b9d2420468c29f812074d3f
• overlay applies with --fuzz=0
• bin/sync-aur sunshine is idempotent
• x86_64 evaluation emits _use_cuda=true, makedepends_x86_64 = cuda, and the CUDA optdepend
• aarch64 evaluation emits _use_cuda=false with no generic CUDA dependency
• makepkg --verifysource passed for the pinned source and both existing backports

A complete CUDA compilation is left to the repository x86_64 builder; the current toolkit is a 4.71 GiB installed dependency. The source, architecture selection, dependency metadata, and overlay regeneration gates all pass locally.