[starcatmeow]

Cache the Windows flake's zstd-compressed qcow2 in GitHub Actions, keyed by the image repository's main commit SHA. A hit skips Nix installation and the image build; a miss builds that exact revision. Save the image immediately after the build so later e2e failures do not discard the cache. Set KVM permissions independently of Nix installation.

Address guest setup problems and missing diagnostics found while debugging the Windows job:

• Task Scheduler defaults to BelowNormal priority. Run Nylon at Normal priority so its control plane is not unnecessarily deprioritized during WinRM activity.
• Set DisableForceUnload in the disposable guests to avoid COM/WinRM failures when a shell closes while another process still holds the user's registry hive.
• Capture Linux stderr alongside stdout; Nylon's logs otherwise produced an empty artifact.

Validation:

• YAML parsing, shell syntax, git diff checks and actionlint passed (ignoring pre-existing setup-go@v4 warnings in unchanged jobs).
• Complete Windows/Linux topology passed on two shared host CPUs in 193.982s, including all four directions with three replies each.
• Read back Normal process priority and DisableForceUnload=1 through fresh WinRM sessions.
• All five jobs passed in https://github.com/encodeous/nylon/actions/runs/41393201151, including the Windows e2e and saving the compressed image to GitHub Actions cache.