chore(performance): add reproducible benchmark harness

[stacknil]

Summary

• add scripts/benchmark-performance-envelope.ps1 to reproduce the performance envelope workload
• generate sanitized 1k/10k/100k syslog corpora under build/performance-envelope
• record measured runs, summary JSON, platform metadata, elapsed time, and peak working set
• update docs/performance-envelope.md with reproduction commands and refreshed script output

Validation

• pwsh -File scripts/benchmark-performance-envelope.ps1 -LineCounts 1000 -Runs 1 -WarmupRuns 0 -SkipBuild
• pwsh -File scripts/benchmark-performance-envelope.ps1 -SkipBuild
• ctest --test-dir build -C Release --output-on-failure
• git diff --cached --check
• privacy/secret scan on changed files