Archive hit concurrency benchmark reports

[bjmeetsfo]

Summary

• add JSON archive output to the hit concurrency benchmark
• add a validator for hit concurrency report archives
• document the report path for memory-hit scale evidence

Validation

• cargo fmt -- --check
• cargo check --locked --no-default-features --examples -q
• cargo run --release --no-default-features --example hit_concurrency_bench -- --json-output /tmp/matrixcache-public-hit-concurrency.json
• python3 tools/validate_hit_concurrency_report.py /tmp/matrixcache-public-hit-concurrency.json
• tools/validate_hit_concurrency_report.py /tmp/matrixcache-public-hit-concurrency.json
• cargo test --locked --no-default-features "concurrency" -- --test-threads=1
• python3 -m py_compile tools/validate_backend_report.py tools/compare_backend_reports.py tools/emit_backend_operator_log.py tools/gen_metrics.py tools/validate_grafana_dashboard.py tools/validate_batch_read_report.py tools/validate_batch_concurrency_report.py tools/validate_hit_concurrency_report.py
• python3 tools/validate_grafana_dashboard.py
• git diff --check
• cargo test --locked --no-default-features -q