docs(performance): add local envelope benchmark

[stacknil]

Summary

• add docs/performance-envelope.md with local 1k/10k/100k-line benchmark results
• record benchmark method, workload shape, platform, elapsed time, and peak working set
• link the performance envelope from README and reviewer path

Benchmark headline

• 1k lines: 44.66 ms median, 3.10 MB peak working set
• 10k lines: 104.01 ms median, 13.82 MB peak working set
• 100k lines: 635.69 ms median, 99.77 MB peak working set

Validation

• cmake --build build --config Release
• ctest --test-dir build -C Release --output-on-failure
• git diff --check
• privacy/secret scan on changed docs