feat: add bitpacked v2 serialization

[mhk197]

Serialize varying widths under fastlanes.bitpacked_v2 with width and offset children and bounded metadata. Uniform arrays retain the original v1 wire contract. Add recursive round trips, compressed-layout execution, and cross-format rejection tests; CUDA support remains uniform-only.

Part 6/9 of the bitpacked-v2 stack (542 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 24 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 degrade performance by 2.58%

⚠️ 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

⚡ 4 improved benchmarks
❌ 4 regressed benchmarks
✅ 2245 untouched benchmarks
⏩ 218 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	WallTime	arrow_checked_add_u32_neon[16384]	12.3 µs	20.3 µs	-39.52%
❌	Simulation	decompress[u64, (4000, 1024)]	71.2 µs	86.6 µs	-17.79%
❌	WallTime	dbp_split_kernel_mixed_null_neon[(I256, 1024)]	1.9 µs	2.2 µs	-12.26%
❌	Simulation	allocate_drop_arrow[0]	462.3 ns	516.4 ns	-10.49%
⚡	Simulation	decode_primitives[f32, (1000, 512)]	64.2 µs	42 µs	+52.92%
⚡	WallTime	mul_u32_nonnull_avx512	6.3 µs	5.6 µs	+11.87%
⚡	Simulation	allocate_drop_bytes[0]	581.3 ns	527.2 ns	+10.27%
⚡	Simulation	decompress[u64, (4000, 4)]	139.5 µs	126.7 µs	+10.1%

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-06-v2-wire (db2c61b) with mk/bitpacked-stack-05-explicit-packing (e1be4ab)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-05-explicit-packing (120d3bd) during the generation of this report, so b0294a3 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩