Decimal Mul/Div kernels

[mhk197]

Adds native Mul and Div execution for decimal arrays, completing the operator set that #8724 started with Add/Sub.

operator	result precision	result scale
Add, Sub	p + 1	s
Mul	2p + 1	2s
Div	p + s + 4	s + 4

Execution

Lanes run at a working width from decimal_numeric_work_dtype, then narrow to the result's own storage width. The two widths are not always the same: Mul's intermediate is the result itself, but Div scales the dividend by 10^result_scale before dividing and so needs room for p + |result_scale| digits. A negative result scale scales the divisor instead.

DecimalOpConstants<W> hoists the per-execution constants — the result-precision bounds and the two division scale factors — out of the lane loop.

Every lane is checked at the working width. DecimalArray does not validate its stored values against the declared precision, so an out-of-precision value can reach a kernel and must not be able to overflow it; test_decimal_value_outside_working_width_errors has depended on that behaviour since #8724. Overflowing the result precision on a valid lane is an error, as is division by zero; invalid lanes never error.

[codspeed-hq]

Merging this PR will degrade performance by 3.86%

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 1923 untouched benchmarks
🆕 4 new benchmarks
⏩ 51 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	decode_varbin[(1000, 2)]	61.8 µs	77.8 µs	-20.61%
⚡	Simulation	slice_dict_tight_loop[10000]	812.1 µs	697.5 µs	+16.43%
🆕	Simulation	mul_decimal_i64_nonnull	N/A	415.4 µs	N/A
🆕	Simulation	div_decimal_i128_nullable	N/A	4.4 ms	N/A
🆕	Simulation	div_decimal_i64_nonnull	N/A	943.6 µs	N/A
🆕	Simulation	mul_decimal_i128_nullable	N/A	2.1 ms	N/A

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/decimal-mul-div (ad8336d) with develop (68e0e58)²}

Footnotes

1. 51 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 develop (5047c16) during the generation of this report, so 68e0e58 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[robert3005]

I think we should make codspeed tests slightly smaller