GH-48906: [C++] Optimize StructArray diffing with field by field comparison

[HyukjinKwon]

Rationale for this change

StructArray diffing was inefficient because it compared all fields together with no early termination. For a 10-field struct, even if the first field differs, it checks all 10 fields.

What changes are included in this PR?

Introduced StructValueComparator that compares struct fields independently with early termination.

Are these changes tested?

Yes, added DiffTest.StructFieldComparison and DiffTest.NestedStructComparison
All existing struct diff tests pass. Plus, I verified this with local benchmark (can be shared if asked).

Are there any user-facing changes?

No behavioral changes but optimization for Array.diff().

• GitHub Issue: [C++] Optimize StructArray diffing with field by field comparison #48906

[github-actions]

⚠️ GitHub issue #48906 has been automatically assigned in GitHub to PR creator.

[rok]

@ursabot please benchmark

[rok]

Benchmark runs are scheduled for commit 2a8a9ab. Watch https://buildkite.com/apache-arrow and https://conbench.arrow-dev.org for updates. A comment will be posted here when the runs are complete.

[rok]

@HyukjinKwon sorry for the noise

[HyukjinKwon]

No please let me know if there is anything to do to verify the fix 🙇‍♂️

[rok]

No please let me know if there is anything to do to verify the fix 🙇‍♂️

I was just testing our benchmarking infra, it's unrelated to your change not much you can do directly on github :).

[conbench-apache-arrow]

Thanks for your patience. Conbench analyzed the 3 benchmarking runs that have been run so far on PR commit 2a8a9ab.

There was 1 benchmark result with an error:

• Pull Request Run on amd64-m5-4xlarge-linux at 2026-01-21 14:29:21Z
  • file-read (Python) with compression=uncompressed, dataset=fanniemae_2016Q4, file_type=feather, output_type=table

There were 2 benchmark results indicating a performance regression:

• Pull Request Run on amd64-c6a-4xlarge-linux at 2026-01-21 14:18:06Z
  • FilterFixedSizeBinaryFilterNoNulls (C++) with params=524288/2/8, source=cpp-micro, suite=arrow-compute-vector-selection-benchmark
  • FilterInt64FilterNoNulls (C++) with params=524288/2, source=cpp-micro, suite=arrow-compute-vector-selection-benchmark

The full Conbench report has more details.