perf: optimize scatter with type-specific specialization (#20498)

## Which issue does this PR close?

Related to #11570 (scatter optimization suggested in
#19994 (comment))

## Rationale for this change

Profiling shows scatter consumes 50%+ of elapsed time in the "10%
zeroes" divide-by-zero protection benchmark. The current implementation
uses the generic MutableArrayData path for all array types.

## What changes are included in this PR?

Apply the same type-specific specialization strategy used in arrow-rs
filter.rs to scatter:

- Selectivity-based iteration: set_slices() for high selectivity,
set_indices() for low selectivity
- Type-specific dispatch via downcast_primitive_array! for primitives,
boolean, bytes, byte views, dictionary, etc.
- Fast paths for all-true and all-false masks
- Fallback to MutableArrayData for unsupported types

## Are these changes tested?

Existing 4 scatter tests preserved. Additional tests will be added.

## Are there any user-facing changes?

No. Public API signature is unchanged.

Author: CuteChuanChuan