benchmark(run): measure vix run performance

[GaspardKirira]

Summary

Add reproducible benchmarks for the main vix run execution paths.

Scenarios

• cached standard-library script;
• uncached standard-library script;
• cached Vix script;
• uncached Vix script;
• CMake fallback script;
• clean project run;
• no-op project run;
• watch rebuild and restart.

Metrics

• planning duration;
• dependency-resolution duration;
• configuration duration;
• compilation duration;
• process startup duration;
• total time to first runtime output.

Acceptance criteria

• Benchmark fixtures are versioned.
• Cold and warm runs are separated.
• Results include machine information.
• Regressions can be compared between commits.
• Benchmarks avoid terminal-rendering overhead.