chore: remove cross-implementation benchmarks and rewrite README

[gargiulofrancesco]

Summary

Repo maintenance ahead of the next release: removes the standalone
cross-implementation benchmarking setup and its tooling, rewrites the README,
and fixes CI. No public API or runtime behavior changes.

Changes

Removed

• Cross-implementation benchmark harness under benchmarks/onpair-bench/
  (Python corpus/runner, C++ bench, standalone Rust bench) and its git submodule.
• Standalone TPC-H example (examples/bench_tpch.rs).
• Python project tooling (pyproject.toml, uv.lock).
• The in-repo Rust benchmarks under benches/ (tpch, clickbench) are retained.

Docs

• Rewrite README.md.
• Minor doc-comment fixes (ClickBench column selection; default-seed docs).

CI

• Install the pinned Rust toolchain in the setup action (drop the reference to a
  nonexistent cache step).

Tests

• Stop asserting the specific default seed value in config tests.

Notes

• No public API or behavior change.
• Quick-start example verified end-to-end against the published onpair 0.1.1 crate.

[codspeed-hq]

Merging this PR will improve performance by 38.92%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 8 improved benchmarks
✅ 24 untouched benchmarks
⏩ 2 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	decompress_all[12]	749 µs	465.4 µs	+60.93%
⚡	WallTime	decompress_all[16]	764 µs	501.6 µs	+52.31%
⚡	WallTime	decompress_all[("p_name", 12)]	1,137.5 µs	769.9 µs	+47.75%
⚡	WallTime	decompress_all[("l_comment", 12)]	16.9 ms	11.7 ms	+44.29%
⚡	WallTime	decompress_all[("o_comment", 12)]	15.1 ms	10.7 ms	+41.71%
⚡	WallTime	decompress_all[("p_name", 16)]	936.5 µs	697.3 µs	+34.3%
⚡	WallTime	decompress_all[("o_comment", 16)]	14.9 ms	12.4 ms	+20.04%
⚡	WallTime	decompress_all[("l_comment", 16)]	15.8 ms	13.6 ms	+16.24%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing chore/repo-maintenance (e4a24c4) with develop (5534b59)}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

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