Add read scale report runner

[bjmeetsfo]

Summary

• add a scale-report runner for read/retrieval cache benchmarks
• validate batch read, batch concurrency, hit concurrency, and read-path JSON reports in one command
• document the runner in the benchmark list

Validation

• cargo fmt -- --check
• python3 -m py_compile tools/run_scale_reports.py tools/validate_batch_read_report.py tools/validate_batch_concurrency_report.py tools/validate_hit_concurrency_report.py tools/validate_read_path_report.py
• tools/run_scale_reports.py --output-dir /tmp/matrixcache-public-scale-reports
• manifest validation for /tmp/matrixcache-public-scale-reports/matrixcache-scale-report-manifest.json
• cargo check --locked --no-default-features --examples -q
• git diff --check
• cargo test --locked --no-default-features -q