feat(fsst): Compute uncompressed size without decoding

[lwwmanning]

Summary

FSST already stores each value’s uncompressed length. This adds an FSST-specific UncompressedSizeInBytes aggregate that calculates the canonical size without decoding string payloads.

Changes

The kernel reads only the lengths child and accounts for canonical views and validity. It rejects negative lengths and arithmetic overflow. Tests compare results with materialized UTF-8 and binary arrays. They cover nullable, all-null, and sliced inputs.

🤖 Generated with Codex

[codspeed-hq]

Merging this PR will improve performance by 14.57%

⚠️ Unknown Walltime execution environment detected

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⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
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⚡ 3 improved benchmarks
✅ 2178 untouched benchmarks
⏩ 206 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	decompress[u64, (4000, 1024)]	85.7 µs	70.4 µs	+21.81%
⚡	Simulation	allocate_drop_bytes[0]	520.2 ns	466 ns	+11.62%
⚡	WallTime	mul_u32_nonnull_avx512	6.3 µs	5.7 µs	+10.61%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing wm/fsst-uncompressed-size (655c3dd) with develop (dab1684)}

Footnotes

1. 206 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. ↩