Document release metadata and gate benchmarks

[SaridakisStamatisChristos]

Summary

• fill out the project metadata with maintainer details and release references, including a changelog
• document serialization compatibility guarantees alongside a versioned magic header
• add a benchmark validation script and wire CI to enforce performance thresholds while publishing a summary artifact

Testing

• python -m pytest -q

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https://chatgpt.com/codex/tasks/task_e_68e607bc75ac8333a6b46efa1ba17e88

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
kll_sketch/kll_sketch.py	80.00%	1 Missing ⚠️

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