refactor(array): give canonical decimals one integer child - #10230

## Summary

- Closes #10750
- Replaces #10746

The docs on `Validity::cast_nullability` read as if every `CastReduce`
rule that removes nullability must call
`Validity::trivially_cast_nullability` and pair with a `CastKernel`.
Most push-down rules (FoR, ALP, ALP-RD, ZigZag, RLE, RunEnd,
DateTimeParts, DecimalByteParts, Chunked, Sparse, Extension) instead
cast the child that holds their validity, and the cast of that child
already fails on nulls, so no result is wrong. Guarding each rule, as
#10746 does for DecimalByteParts, adds a kernel per encoding and delays
the rewrite until execution without changing any result.

The guard in #10746 comes from the closed #10230, where a canonical
decimal held a lazy child that a cast to canonical did not execute, by
way of the FX2 notes in #10633. `develop` keeps one buffer per decimal,
so that reason no longer applies.

## Changes

`CastReduce` now owns the contract and documents both forms of rule. A
rule that builds a new `Validity` keeps the guard and the kernel, and a
rule that pushes the cast into its children needs neither when the nulls
of those children are exactly the null rows of the array. `Dict` is the
documented exception, because its values can hold nulls that no code
references. No new tests are needed: every push-down encoding listed
above already runs `test_cast_conformance` on a partially valid array,
and that suite asserts that the cast to non-nullable fails.

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