perf: explore bulk copies for no-null fixed-width Arrow writes #5299

What is the problem the feature request solves?

ArrowWriter.writeColNoNull still copies fixed-width columns one value at a time with setValueUnsafe. This avoids Arrow's per-value capacity check, but retains a getter call and setter call for every value.

A bulk copy into the Arrow data buffer could remove that per-row dispatch for eligible Spark column vectors. This was identified during review of #5046.

Describe the potential solution

Investigate a specialized no-null path for fixed-width types whose Spark and Arrow physical layouts match:

• copy the contiguous value region into BaseFixedWidthVector.getDataBuffer;
• initialize the destination validity bitmap in bulk;
• preserve the existing per-value path for unsupported vectors and layouts;
• handle sliced input offsets, dictionaries, endianness, and both on-heap and off-heap inputs;
• benchmark the specialized path across representative types and batch sizes before adopting it.

There are API constraints to resolve first:

• OffHeapColumnVector.valuesNativeAddress exposes the values address but is annotated @VisibleForTesting;
• OnHeapColumnVector keeps its primitive arrays private, while its public bulk getters allocate new arrays;
• ColumnarArray keeps its backing vector and offset private, so the current writer API does not expose the information needed for a direct copy;
• booleans are byte-per-value in Spark but bit-packed in Arrow, and decimals/intervals do not generally share a directly copyable layout.

Avoid reflection or private-field access that would be brittle across supported Spark versions. The issue should only proceed if benchmarks show that a stable source-access approach beats the current loop.

Additional context

• Parent optimization: perf: improve ArrowWriter performance for fixed-length vectors #5046
• Review summary: perf: improve ArrowWriter performance for fixed-length vectors #5046 (review)
• Inline suggestion: perf: improve ArrowWriter performance for fixed-length vectors #5046 (comment)