Use bulk writes instead of per-element writes in vector serialization

[r-devulap]

Replace inefficient element-by-element write loops with bulk write operations in MemorySegmentVectorProvider:

• writeFloatVector: Extract underlying float array and use writeFloats() instead of looping with writeFloat()
• writeByteSequence: Extract underlying byte array and use write() instead of looping with writeByte()

With highway as the SIMD backend #668 , the scalar writes show up as a bottleneck when constructing index on a AWS instance x8i.24xlarge (2-socket 96 core Intel GNR) .

Dataset	Index Build Time Before (s)	Index Build Time After (s)
openai-1536-1m	105.59	45.22
openai-3072-1m	164.63	102.75

[github-actions]

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[tlwillke]

writeFloatVector is fine. writeByteSequence has a bug. You are ignoring slice offsets. MemorySegmentByteSequence supports slicing, but .heapBase().get() returns the original unsliced array. I have suggested a change that fixes this and added minimal testing.

Once this is addressed and the tests pass, it is good to go.

On performance, while I appreciate the end-to-end benchmarking and consider it necessary, I would also like to see a unit-level microbenchmark isolating writeFloatVector and writeByteSequence to quantify the exact bulk-write speedup.

[tlwillke]

Suggested change

	{
	for (int i = 0; i < sequence.length(); i++)
	out.writeByte(sequence.get(i));
	byte[] data = (byte[]) ((MemorySegmentByteSequence) sequence).get().heapBase().get();
	out.write(data, 0, sequence.length());
	}
	{
	java.nio.ByteBuffer bb = ((MemorySegmentByteSequence) sequence).get().asByteBuffer();
	out.write(bb.array(), bb.arrayOffset(), bb.remaining());
	}

[r-devulap]

Thanks for pointing that out. I mistakenly assumed CI would catch this bug. I’ll fix it.