Reduce per-call allocations in GraphSearcher.searchOneLayer and FusedPQDecoder

[eolivelli]

Summary

Allocation profiling of OnDiskGraphIndexCompactor workloads (async-profiler alloc mode, 30 s) showed two hotspots accounting for ~87% of all allocations on the compaction ForkJoin workers. This PR removes both.

Site	Before	What it allocated
GraphSearcher.searchOneLayer (lines 483 + 488)	61.9% of allocs	NeighborProcessor lambda + visited::add method reference, allocated per loop iteration via DirectMethodHandle.allocateInstance
FusedPQDecoder.newDecoder → <init> → VectorUtil.sub	24.8% of allocs	new ArrayVectorFloat(new float[dim]) for the centered query on every decoder construction (one per search)

GraphSearcher.searchOneLayer

• Hoist scoreTracker to an instance field so the NeighborProcessor lambda needs to capture only this.
• Make the NeighborProcessor a permanent instance field, allocated once in the constructor.
• Make the IntMarker for visited a final field bound once at construction (this.visitedAdder = visited::add).
• Call site now uses the two fields instead of building fresh callbacks per iteration.

FusedPQDecoder / FusedPQ

• Add a ThreadLocal<VectorFloat<?>> reusableCenteredQuery in FusedPQ, mirroring the existing reusableResults / reusableNeighborCodes / pqCodeScratch pattern.
• Plumb the buffer through FusedPQ.approximateScoreFunctionFor → FusedPQDecoder.newDecoder and the three subclass constructors.
• Replace VectorUtil.sub(query, center) (allocates) with a new VectorUtil.subInto(dest, lhs, rhs) helper that writes the difference into the thread-local scratch.

The centered-query buffer is read-only after construction (just feeds VectorUtil.calculatePartialSums), so a thread-local lifetime is safe — and FusedPQDecoder is already constructed concurrently from multiple ForkJoin workers.

Test plan

• ./mvnw -pl jvector-base,jvector-twenty,jvector-native -am compile — all modules compile.
• ./mvnw -pl jvector-tests -am test — 275 tests pass, 0 failures, 0 errors, 2 skipped. Includes TestOnDiskGraphIndexCompactor, TestFusedGraphIndex, TestProductQuantization, TestCompressedVectors, TestReconstructionError, TestOnDiskGraphIndex, Test2DThreshold, TestConcurrentReadWriteDeletes, etc.
• Re-profile the HerdDB indexing workload with asprof -e alloc and confirm the two allocation sites drop to near zero.

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