Benchmark provenance

Benchmark runs now write a consolidated report.json and a per-machine index.json under:

bench/{machine}/{yyyy-MM-dd}/{HH-mm}/

The report records the commit, runtime, BenchmarkDotNet version, effective document count and data source fingerprints. This makes copied runs and shared benchmark folders easier to compare.

When running outside a Git checkout, pass the source metadata explicitly:

.\scripts\benchmark.ps1 -SourceCommit abc123 -SourceRef main -SourceManifest manifest.json

See Benchmarking.