Reduce engines CI duplication and pull-request latency

Optimize the .NET CI workflow using observed job timings. Remove duplicate restores/builds, avoid rebuilding test projects during execution, limit SDK installation to versions actually required, and move expensive release-only Native AOT/audio validation off the pull-request critical path while retaining it on protected-branch pushes and in the client release workflow. Preserve allocator, Vulkan, client, graphics, production-feature, engine, navigation, and release gates.\n\nAcceptance criteria:\n- Client projects restore/build once before routine managed tests.\n- Allocator and Vulkan tests reuse the validated build where configuration permits.\n- Native audio and Native AOT do not extend ordinary pull-request jobs but remain validated after merge and during tagged releases.\n- Engine CI restores the shared graph once, retains the scoped zero-warning engine build, and avoids redundant dependency restore before the unit-test build.\n- CI configuration is syntactically valid and equivalent commands pass locally where practical.\n\nPull request: #2656 https://git.project-rk.com/prk/engines/pulls/2656