feat: add bitpacked v2 serialization

[mhk197]

Serialize arrays with differing chunk widths under fastlanes.bitpacked_v2. The format stores one offsets child containing num_chunks + 1 byte boundaries; widths are derived from adjacent differences. Metadata contains only bounded array and patch information.

Uniform arrays retain the v1 wire format, which omits the offsets child and stores one width in metadata. Empty arrays use width zero because there are no chunks from which to derive a width. Deserialization still accepts legacy empty arrays with a nonzero width.

Add recursive round trips, execution with compressed offsets, validation of malformed compressed boundaries, and rejection of mismatched format IDs and child layouts. Extract shared wire-format helpers while preserving the v1 validation order. CUDA support remains limited to uniform widths.

Validation: 403 FastLanes/BtrBlocks tests passed (1 skipped).

[codspeed]

Merging this PR will improve performance by 79.37%

⚠️ 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.

⚠️ 4 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

⚡ 1 improved benchmark
✅ 2231 untouched benchmarks
⏩ 329 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	take_fsl_random[128, 10]	58.6 µs	32.7 µs	+79.37%
⚠️	Simulation	take_fsl_u32_random[256, 10]	< 1 ns	< 1 ns	N/A
⚠️	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

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_{Comparing mk/bitpacked-stack-06-v2-wire (60ce538) with mk/bitpacked-stack-05-explicit-packing (0e28beb)²}

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. ↩

2. No successful run was found on mk/bitpacked-stack-05-explicit-packing (82c7619) during the generation of this report, so a3ce2bd was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩