perf: surface optimization overhead diagnostics

[akothari-godaddy]

Summary

• add overhead diagnostics to perf JSON output
• report optimization p50/p95/p99, slow request percentage, per-stage totals/percentiles, and top slow requests
• update text report and recommendations to point at the slowest stage and HEADROOM_COMPRESSION_TIMEOUT_SECONDS when optimization is consistently slow

Verification

• python -m py_compile headroom/perf/analyzer.py tests/test_cli_perf_format.py
• pytest tests/test_cli_perf_format.py could not run locally because pytest is not installed in this Python environment

[github-actions]

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