Eliminate float[] and ByteBuffer allocations in compaction inline-record path

[eolivelli]

Summary

Profiling a HerdDB indexing workload showed CompactWriter.writeInlineNodeRecord accounting for 24% of alloc-event bytes:

Site	% of total alloc	Cause
writeInlineNodeRecord:222	21.4%	pq.encodeTo → VectorUtil.sub(vector, globalCentroid) → fresh ArrayVectorFloat (float[]) per neighbor
writeInlineNodeRecord:248	2.4%	bwriter.cloneBuffer() → ByteBuffer.allocate(recordSize) per record

ArrayVectorFloat.<init> was the single largest allocator in the whole profile (20.6% self) and fully attributable to the PQ encode call inside compaction.

This PR removes both allocations on this hot path.

Changes

• ProductQuantization.encodeTo(vector, scratch, dest) — new 3-arg overload that, when a global centroid is configured, uses VectorUtil.subInto to write the centered vector into a caller-supplied scratch buffer instead of allocating a fresh one. The existing 2-arg overload keeps its allocating behavior (it's the VectorCompressor.encodeTo implementation; all other callers stay unchanged).
• OnDiskGraphIndexCompactor.processBaseNode — passes scratch.tmpVec as the scratch buffer. tmpVec is per-worker, dimension-sized, and already dead by the time retainDiverse returns, so no extra allocation is needed.
• CompactWriter.setInlineChannel(FileChannel) + direct positional writes — the level-0 FileChannel is plumbed into CompactWriter, and writeInlineNodeRecord writes the per-thread record buffer directly to disk via FileChannel.write(ByteBuffer, long). This positional API is thread-safe and lets us drop ByteBufferIndexWriter.cloneBuffer and the WriteResult.data field; the level-0 consumer in compactLevels is now a no-op since records hit disk inside the worker.

The on-disk byte layout at level 0 is unchanged, so existing compacted indexes remain readable.

Test plan

• mvn -pl jvector-tests -am test — 275 tests pass, 0 failures / 0 errors (2 pre-existing skipped).
• mvn -pl jvector-tests -am -Dtest='io.github.jbellis.jvector.graph.disk.*Test*,TestProductQuantization,TestPQRetrainer*' test — focused on the touched code paths (42 tests pass).
• Re-profile the HerdDB indexing workload and confirm ArrayVectorFloat.<init> drops to near zero in the compactor stack and HeapByteBuffer.<init> drops the 2.3% attributable to cloneBuffer.

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