refactor(runend): clarify filter implementation

[connortsui20]

Summary

Cleans up RunEnd filtering without changing its selection policy or output.

Changes

Names and documents the take thresholds, clarifies the fast path and run-end reconstruction, and preserves the bulk popcount and branchless write paths. The optimized x86-64-v3 and x86-64-v4 LLVM IR and assembly were checked, and the generated filter path is identical before and after.

[codspeed-hq]

Merging this PR will improve performance by 19.24%

⚠️ 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

⚡ 6 improved benchmarks
✅ 2106 untouched benchmarks
⏩ 106 skipped benchmarks¹
🗄️ 4 archived benchmarks run²

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	arrow_checked_add_u32_neon[16384]	20.4 µs	13.6 µs	+50.36%
⚡	WallTime	arrow_checked_add_u32_avx512[16384]	21.3 µs	17.6 µs	+20.52%
⚡	WallTime	words_gather_scalar_avx2[65536]	9.5 µs	8.3 µs	+14.58%
⚡	Simulation	cold_misaligned[(16, 64)]	391.3 µs	345.1 µs	+13.37%
⚡	Simulation	compress_fsst[(500, 64, 4)]	488.1 µs	441.6 µs	+10.52%
⚡	Simulation	compress_fsst[(500, 64, 8)]	524.9 µs	475.1 µs	+10.48%

Tip

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

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_{Comparing ct/runend-filter-cleanup (83e5caf) with develop (ee1ac25)}

Footnotes

1. 106 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. ↩

2. 4 benchmarks were run, but are now archived. If they were deleted in another branch, consider rebasing to remove them from the report. Instead if they were added back, click here to restore them. ↩