Decimal arithmetic dtypes and widening rescale cast

[gatesn]

Binary::return_dtype derives Hive-style result types for decimal + - * / (mirroring arrow-arith's execution-time rules, with precision saturation at the physical width), instead of erroring at plan time. The decimal cast kernel gains widening rescale so expression coercion can align decimal operand types.

[codspeed-hq]

Merging this PR will degrade performance by 27.11%

⚠️ 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.

❌ 3 regressed benchmarks
✅ 1529 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	chunked_bool_canonical_into[(1000, 10)]	20.2 µs	35.5 µs	-43.03%
❌	Simulation	chunked_varbinview_canonical_into[(1000, 10)]	162 µs	198.1 µs	-18.25%
❌	Simulation	chunked_varbinview_into_canonical[(1000, 10)]	177.2 µs	213.1 µs	-16.86%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing ngates/decimal-arithmetic-dtypes (2c37139) with develop (3d7bbfb)}