perf: fuse bitpacked chunk encoding

[mhk197]

Fuse histogramming, width selection, exception gathering, and packing while each chunk is in cache. Retain the multipass implementation as a test oracle and verify equivalence, including a short trailing chunk whose padded packed size exceeds its raw size.

Part 8/9 of the bitpacked-v2 stack (265 added + removed lines). Merge in order; each draft targets the preceding branch, starting from develop.

1 → 2 → 3 → 4 → 5 → 6 → 7 → 8 → 9

Validation: All 55 focused plugin, serde, and chunk-layout tests passed on this commit, including the regression that rejects direct vtable serde. Changed Rust files were formatted with nightly; git diff --check passed.

[codspeed]

Merging this PR will regress 5 benchmarks

⚠️ 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
❌ 5 regressed benchmarks
✅ 2242 untouched benchmarks
⏩ 218 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	random_i16[0.95]	81.5 µs	99.7 µs	-18.28%
❌	WallTime	filtered_sink_i64_avx512[OneNullInEight]	23 µs	26.4 µs	-13.01%
❌	WallTime	words_gather_scalar_avx2[1024]	129 ns	147 ns	-12.24%
❌	WallTime	words_gather_scalar_avx2[65536]	8.3 µs	9.3 µs	-11.66%
❌	WallTime	words_gather_dispatch_avx2[65536]	1.2 µs	1.3 µs	-10.18%
⚡	WallTime	arrow_checked_add_u32_neon[16384]	20.4 µs	12.3 µs	+66.28%
⚡	Simulation	decode_primitives[f32, (1000, 512)]	64.2 µs	42 µs	+52.92%
⚡	Simulation	random_i8[0.5]	96 µs	72.4 µs	+32.68%
⚡	Simulation	decompress[u64, (4000, 1024)]	86.9 µs	71 µs	+22.33%
⚡	WallTime	dbp_split_kernel_mixed_null_neon[(I256, 1024)]	2.2 µs	1.9 µs	+13.78%
⚡	WallTime	filtered_owned_i64_avx2[OneNullInEight]	25.7 µs	22.6 µs	+13.65%

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 (910328c) with mk/bitpacked-stack-07-width-selection (60c53a1)²}

Footnotes

1. 218 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. No successful run was found on mk/bitpacked-stack-07-width-selection (3e5dc30) during the generation of this report, so f592b06 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩