perf: optimize greatest in datafusion-functions

[andygrove]

This PR was created by an LLM as a draft PR. I will mark it as ready for review after human review.

Which issue does this PR close?

N/A — autonomous exploratory PR.

Rationale for this change

Vectorize greatest's nullable comparison path — replace the per-row boxed make_comparator with cmp::gt_eq plus bitwise null masking, keeping the comparator only for floats where NaN ordering differs.

What changes are included in this PR?

Vectorize greatest's nullable comparison path — replace the per-row boxed make_comparator with cmp::gt_eq plus bitwise null masking, keeping the comparator only for floats where NaN ordering differs.

Are these changes tested?

Correctness: unit tests + seeded differential fuzz (bit-identical Arrow output vs main).

Benchmark (criterion):

• greatest_utf8_nullable: 9.023% faster (base 10651ns -> cand 9690ns)
• greatest_i64_nullable: 21.744% faster (base 7310ns -> cand 5721ns)
• greatest_i64_nullable_3_args: 21.667% faster (base 14078ns -> cand 11027ns)
• greatest_i64_no_nulls: 1.398% faster (base 3163ns -> cand 3119ns)

Full criterion output:

greatest_i64_nullable   time:   [5.7071 µs 5.7111 µs 5.7164 µs]
                        change: [−21.848% −21.744% −21.617%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  1 (1.00%) high mild
  4 (4.00%) high severe

greatest_i64_no_nulls   time:   [3.1184 µs 3.1210 µs 3.1238 µs]
                        change: [−1.4889% −1.3979% −1.3102%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 3 outliers among 100 measurements (3.00%)
  2 (2.00%) high mild
  1 (1.00%) high severe

greatest_i64_nullable_3_args
                        time:   [11.012 µs 11.022 µs 11.033 µs]
                        change: [−21.773% −21.667% −21.583%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 13 outliers among 100 measurements (13.00%)
  12 (12.00%) high mild
  1 (1.00%) high severe

greatest_utf8_nullable  time:   [9.6840 µs 9.6950 µs 9.7074 µs]
                        change: [−9.2000% −9.0226% −8.8658%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high severe

Are there any user-facing changes?