feat[gpu]: widen decimals for Arrow device export

[0ax1]

Arrow Decimal128/Decimal256 schemas require fixed 16/32-byte value buffers, while Vortex decimals may use narrower storage. Add a CUDA widening kernel for Arrow Device export and cover compact, wide, nullable, and empty decimal cases.

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

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⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 1264 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	chunked_varbinview_opt_canonical_into[(1000, 10)]	225.4 µs	188.1 µs	+19.84%
❌	Simulation	chunked_varbinview_canonical_into[(100, 100)]	273.1 µs	307.8 µs	-11.27%

Tip

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_{Comparing ad/arrow-device-decimal (867a83f) with develop (c005aae)}

[onursatici]

can this end up truncating? I think it is currently possible in vortex to do this:

  let array = DecimalArray::from_iter(
      [i256::from_parts(0, 1)], // 2^128, so does not fit into i128
      DecimalDType::new(38, 0), // this is normally i128
  );

when exporting this array we would pick the i256 -> i128 kernel and truncate the values without checking for overflow.

probably the right fix is for vortex to reject constructing such arrays