GH-3464 Improve DeltaByteArrayWriter.writeBytes to avoid unnecessary allocation and scalar prefix comparison

[arouel]

Rationale for this change

DeltaByteArrayWriter.writeBytes() is on the hot path for DELTA_BYTE_ARRAY encoding and had avoidable overhead:

1. Per-value allocation from getBytes().
   getBytes() always creates a new array. For prefix comparison this copy is unnecessary.

2. Scalar prefix scan.
   The byte-by-byte loop is replaced with Arrays.mismatch(...), which maps to optimized JVM intrinsics.

In profiling (custom JFR benchmark on a large merge workload), this method was a top allocation hotspot before the change.

What changes are included in this PR?

In DeltaByteArrayWriter.writeBytes():

• v.getBytes() -> v.getBytesUnsafe() for read-only prefix comparison.
• Manual prefix loop -> Arrays.mismatch(previous, 0, length, vb, 0, length).
• previous = vb -> previous = v.isBackingBytesReused() ? v.getBytes() : vb
  (defensive copy only when the backing bytes may be reused by the caller).

This preserves semantics while removing unnecessary allocations in the common case.

Benchmark signal (custom, directional)

On a custom JFR-profiled merge workload (180M rows, 4 binary columns), this change significantly reduced allocations and lowered CPU attributed to this path.
(Results are workload/JDK dependent and provided as directional evidence.)

Are these changes tested?

Yes.

• Existing coverage: TestDeltaByteArray round-trip tests (including skip/skipN/reset paths).
• Added regression test: testReusedBackingArrayRegression in TestDeltaByteArray to verify correctness when the same mutable backing array is reused across writes.

Are there any user-facing changes?

No API or format changes. This is a transparent performance optimization; encoded data remains compatible/interchangeable.

Closes #3464