refactor: store bitpacked chunk widths in a child

[mhk197]

Replace the in-memory scalar width with a non-nullable u8 child. Uniform encoders use a constant child; bulk kernels prepare the layout once per operation. V1 serialization omits the child. Update callers and CUDA's Rust adapters; CUDA continues to accept only uniform widths.

Part 3/9 of the bitpacked-v2 stack (700 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 6 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.

GPU execution was not tested. CUDA validation and workspace-wide all-feature Clippy were previously blocked by the nvCOMP 5.1 SDK download in this environment.

[codspeed]

Merging this PR will regress 2 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

⚡ 3 improved benchmarks
❌ 2 regressed benchmarks
✅ 2248 untouched benchmarks
⏩ 218 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	bitpacked_compress_u32	36.5 µs	47.1 µs	-22.52%
❌	Simulation	random_i16[0.8]	79.4 µs	97.8 µs	-18.85%
⚡	WallTime	arrow_checked_add_u32_neon[16384]	20.4 µs	12.3 µs	+66.78%
⚡	Simulation	decode_primitives[f32, (1000, 512)]	64.2 µs	42 µs	+52.92%
⚡	Simulation	random_i8[0.5]	96 µs	72.4 µs	+32.6%

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-03-width-child (d740f8f) with mk/bitpacked-stack-02-cpu-layout (9737c00)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-02-cpu-layout (7390ffe) during the generation of this report, so 64a967c was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩