Linux / cross-platform CI benchmarks

[SolidRegardless]

Summary

Set up cross-platform CI pipelines (GitHub Actions) that automatically run benchmarks on Linux and report results, ensuring the project works reliably beyond Windows.

Motivation

The project claims cross-platform portability via .NET 8 and ILGPU, but all current testing and benchmarking is done on Windows. Without CI validation on Linux, regressions or platform-specific issues could go unnoticed. Automated benchmark runs also provide a performance history that can catch regressions early and give contributors confidence that their changes don't break anything.

Acceptance Criteria

• Add a GitHub Actions workflow that builds and runs the benchmark suite on Ubuntu (latest LTS)
• CI should run on push to main and on pull requests
• CPU-mode benchmarks should always run (no GPU required)
• Results should be captured as CI artifacts (using CSV/Markdown export from CSV / Markdown export #10 if available)
• Build failures and benchmark crashes should fail the CI pipeline
• Optionally: add a Windows CI job for parity
• Optionally: if a GPU runner is available (e.g. self-hosted), run CUDA/OpenCL benchmarks too
• Add a CI status badge to the README

Technical Notes

• GitHub-hosted runners don't have GPUs, so CI will primarily validate CPU-mode correctness and performance
• Use dotnet build and dotnet run in the workflow — no special tooling needed
• Consider caching NuGet packages for faster CI runs
• For GPU CI, a self-hosted runner with NVIDIA drivers would be needed (future enhancement)
• Benchmark results could be posted as PR comments using GitHub Actions for visibility
• This pairs well with CSV / Markdown export #10 (export) for structured result capture