Rationale for this change

ByteStreamSplitValuesReader is the symmetric reader for BYTE_STREAM_SPLIT-encoded FLOAT, DOUBLE, INT32, and INT64 columns. On initFromPage it eagerly transposes the entire page from stream-split layout (elementSizeInBytes separate streams of valuesCount bytes each) back to interleaved layout. The current loop is:

private byte[] decodeData(ByteBuffer encoded, int valuesCount) {
  byte[] decoded = new byte[encoded.limit()];
  int destByteIndex = 0;
  for (int srcValueIndex = 0; srcValueIndex < valuesCount; ++srcValueIndex) {
    for (int stream = 0; stream < elementSizeInBytes; ++stream, ++destByteIndex) {
      decoded[destByteIndex] = encoded.get(srcValueIndex + stream * valuesCount);
    }
  }
  return decoded;
}

Two issues on the hot path:

1. Every read goes through ByteBuffer.get(int) (per-call bounds checks + virtual dispatch through HeapByteBuffer/DirectByteBuffer).
2. The inner stream offset (stream * valuesCount) is recomputed on every iteration even though it depends only on the outer loop.

For a 100k-value FLOAT page that is 400k ByteBuffer.get(int) calls; for DOUBLE/LONG it is 800k.

What changes are included in this PR?

Rewrite decodeData in three steps:

1. Drop down to a byte[] view of the encoded buffer. When encoded.hasArray() is true (the typical case), use the backing array directly with the correct base offset; otherwise copy once with a single get(byte[]) call. Eliminates the per-byte ByteBuffer.get(int) bounds check and virtual dispatch.

2. Specialize loops for the common element sizes (4 and 8). Hoist all stream * valuesCount offsets into local ints (s0..s3 for floats/ints, s0..s7 for doubles/longs) and write each output slot exactly once in a single sequential pass. Reads come from elementSizeInBytes concurrent sequential streams, which modern hardware prefetchers handle well.

3. Generic fallback for arbitrary element sizes (FIXED_LEN_BYTE_ARRAY of any width).

Benchmark

New ByteStreamSplitDecodingBenchmark (100k values per invocation, JDK 18, JMH -wi 5 -i 10 -f 3, 30 samples per row):

Decoded output is byte-identical to before; per-op heap allocation is unchanged.

Are these changes tested?

Yes. All 573 parquet-column tests pass; 51 BSS-specific tests pass (mvn test -pl parquet-column -Dtest='*ByteStreamSplit*'). No new test was added because the decoded bytes are unchanged (covered by existing round-trip and ByteStreamSplitValuesReaderTest tests).

Are there any user-facing changes?

No. Only an internal reader optimization. No public API, file format, or configuration change.

Closes #3505

Symmetric companion to #3504 (writer-side BSS optimization). Part of a small series of focused performance PRs from work in parquet-perf. Previous: #3494, #3496, #3500, #3504.

How to reproduce the benchmarks

The JMH benchmarks cited above are being added to parquet-benchmarks in #3512. Once that lands, reproduce with:

./mvnw clean package -pl parquet-benchmarks -DskipTests \
    -Dspotless.check.skip=true -Drat.skip=true -Djapicmp.skip=true
java -jar parquet-benchmarks/target/parquet-benchmarks.jar 'ByteStreamSplitDecodingBenchmark' \
    -wi 5 -i 10 -f 3

Compare runs against master (baseline) and this branch (optimized).