perf: fuse bitpacked chunk encoding

[mhk197]

Fuse histogramming, width selection, exception gathering, and packing while each chunk is in cache. Convert the temporary width choices into the offsets child once packing is complete.

Retain the multipass implementation as a test oracle and verify identical layouts, packed bytes, exceptions, and decoded values. Include a short trailing chunk whose padded packed size exceeds its raw size.

Validation: 435 FastLanes/BtrBlocks tests passed (1 skipped).

[codspeed]

Merging this PR will regress 1 benchmark

⚠️ 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.

⚠️ 3 benchmarks measured no execution time

Nothing ran under measurement, usually because the compiler removed the code under test. These results are not comparable, so they count as unchanged.

Preventing compiler optimizations

⚠️ 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

⚡ 3 improved benchmarks
❌ 1 regressed benchmark
✅ 2228 untouched benchmarks
⏩ 329 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	take_fsl_u8_random[256, 100]	45.8 µs	98.6 µs	-53.53%
⚡	Simulation	take_fsl_f16_random[256, 100]	141.1 µs	85.4 µs	+65.33%
⚡	Simulation	take_chunked_fsl_sorted[32, 64]	252.3 µs	195.8 µs	+28.84%
⚡	Simulation	take_fsl_random[64, 100]	143.8 µs	124.9 µs	+15.15%
⚠️	Simulation	fixed_16_advancing_ptr_safe[100]	< 1 ns	< 1 ns	N/A
⚠️	Simulation	preverify_advancing_ptr_unchecked[1000]	< 1 ns	< 1 ns	N/A
⚠️	Simulation	preverify_advancing_ptr_unchecked[10000]	< 1 ns	< 1 ns	N/A

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing mk/bitpacked-stack-08-fused-encoder (ab1d8e1) with mk/bitpacked-stack-07-width-selection (d5ffca3)}

Footnotes

1. 329 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. ↩