GH-3520: Cleanup binary write path (DeltaByteArrayWriter copy + FLBA LE wrapper)

[iemejia]

Summary

Resolves #3520.

Two small cleanups in the binary write path.

1. DeltaByteArrayWriter: avoid unconditional copy of input bytes

The first line of writeBytes(Binary v) is:

byte[] vb = v.getBytes();

Binary.getBytes() is contractually required to return a fresh array the caller can keep and mutate. For ByteArrayBackedBinary (the most common case from Binary.fromConstantByteArray() and similar), the implementation does an unconditional Arrays.copyOf of the backing array — but DeltaByteArrayWriter only reads vb and then drops it. The copy is wasted work on every value.

The right call here is v.copy().getBytesUnsafe():

• Binary.copy() is a no-op (return this) for constant Binaries that are already independent of any reused buffer.
• For reused-buffer Binaries (e.g. ByteBufferBackedBinary over a slab being mutated), copy() snapshots them — preserving correctness.
• getBytesUnsafe() then returns the backing array directly without a defensive copy.

For the common ByteArrayBackedBinary case this skips the entire copy. For other implementations the copy still happens but only when it's actually needed for correctness.

2. FixedLenByteArrayPlainValuesWriter: drop the unused LittleEndianDataOutputStream wrapper

Same pattern as #3517 fixed in DeltaLengthByteArrayValuesWriter: the writer wraps its CapacityByteArrayOutputStream with a LittleEndianDataOutputStream that's only used to call Binary.writeTo() — i.e. the LE wrapper adds a layer of dispatch on every value but never uses any LE-specific functionality (writeInt/writeLong/etc.). Binary.writeTo(arrayOut) works directly with the underlying stream.

The trailing out.flush() in getBytes() is also dead — CapacityByteArrayOutputStream doesn't buffer.

Benchmark

BinaryEncodingBenchmark.encodeDeltaByteArray (short strings): roughly +5% to +10% standalone — the per-value getBytes() copy is one of several overheads; this PR removes one of them, stacking with #3517 which removed the suffix-side allocation.

FixedLenByteArrayPlainValuesWriter: code-quality cleanup; removes a per-value layer of dispatch and an unnecessary flush() call. No headline benchmark.

Validation

• parquet-column: 573 tests pass
• Built with -Dspotless.check.skip=true -Drat.skip=true -Djapicmp.skip=true

User-facing changes

None. No public API change. No file format change.

Closes #3520

Part of a small series of focused performance PRs from work in parquet-perf. Previous: #3494, #3496, #3500, #3504, #3506, #3510, #3514, #3517, #3519. Companion benchmarks contribution: #3512.

[arouel]

I address this area already in PR #3465. Can review it and reduce that one to the part which is not addressed there?