perf(buffer): store alignment exponents

[gatesn]

Summary

Store Alignment as its base-2 exponent instead of a usize.

Changes

• Shrink Alignment from 8 bytes to 1 byte.
• Remove Deref and convert to bytes explicitly with as_usize().
• Keep the accepted alignments and serialized exponent unchanged.

[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

⚡ 4 improved benchmarks
❌ 2 regressed benchmarks
✅ 2130 untouched benchmarks
⏩ 164 skipped benchmarks¹
🗄️ 1 archived benchmark run²

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]	13.5 µs	20.3 µs	-33.57%
❌	WallTime	mul_u32_nonnull_avx512	5.6 µs	6.4 µs	-12.1%
⚡	WallTime	arrow_checked_add_u32_avx512[16384]	21.3 µs	17.6 µs	+20.83%
⚡	WallTime	arrow_checked_add_u32_avx2[16384]	21.3 µs	17.7 µs	+20.35%
⚡	Simulation	compact[(1024, 90)]	1.6 µs	1.4 µs	+11.57%
⚡	Simulation	compact[(2048, 90)]	1.6 µs	1.5 µs	+11.11%

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 ngates/buffer-alignment-exponent (79d06ff) with develop (aaed723)}

Footnotes

1. 164 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. 1 benchmark was run, but is now archived. If it was deleted in another branch, consider rebasing to remove it from the report. Instead if it was added back, click here to restore it. ↩