perf: specialize single-column hash-join key equality check

[shehab-ali]

Rationale for this change

Hash-join key-equality verification (equal_rows_arr) checks every candidate row pair produced by a hash-bucket match. The current implementation calls JoinKeyComparator::is_equal per pair, which for each pair:

1. invokes a boxed DynComparator closure (an un-inlinable indirect call)
2. computes a full three-way Ordering, only to discard everything except equal / not-equal.

For single-column, high-fanout joins with near-100% match rates (long string keys are the worst case), that per-pair overhead dominates the join and measurably regresses throughput relative to the previous vectorized approach.

Benchmarking against main on the hj SQL benchmark suite (TPC-H SF10, interleaved runs to cancel drift) showed this cost concentrated in single-column-key queries:

• Q23 (a high-fanout ~28-char string key) ran ~5% slower, with a cluster of other single/low-column joins 6–22% slower.

What changes are included in this PR?

• Add equal_rows_single_col: a specialized fast path for single-column join keys. It downcasts the key array once and runs a monomorphized == loop over the candidate pairs, eliminating the per-pair boxed-closure dispatch and the Ordering computation.
• Covered types: integers, Decimal128, dates, timestamps, boolean, and all Utf8/Binary variants (incl. Utf8View/BinaryView).
• Multi-column keys and unspecialized types fall through unchanged to the existing JoinKeyComparator path. Floats (Float16/32/64) are intentionally excluded so their -0.0/NaN equality semantics remain on the exact same code path as before.
• Null handling is preserved: NullEqualsNothing -> both-null is unequal; NullEqualsNull -> both-null is equal.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.61111% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.90%. Comparing base (1955d5a) to head (b2eef82).
⚠️ Report is 63 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/physical-plan/src/joins/utils.rs	98.61%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24067      +/-   ##
==========================================
+ Coverage   80.75%   80.90%   +0.14%     
==========================================
  Files        1096     1102       +6     
  Lines      373582   376357    +2775     
  Branches   373582   376357    +2775     
==========================================
+ Hits       301686   304482    +2796     
+ Misses      53893    53769     -124     
- Partials    18003    18106     +103     

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