perf(hnsw): reuse visited workspaces

[zhangzhenghao]

Summary

• reuse a pooled generation-marked visited workspace for HNSW traversals
• share one workspace across the layers of each query or graph insertion
• cover dense, sparse, scalar-quantized, RaBitQ, parallel-build, and group-by traversal paths
• add reset, resize, generation-wrap, shrink/grow, and allocation tests

Benchmark

BenchmarkHNSWSearch on linux/amd64:

	main	this PR
B/op	51,440	10,481
allocs/op	42	38

This reduces allocated bytes per search by 79.62% in the benchmark.

Verification

• go test ./... -count=1
• go vet ./...
• go test -race ./... -count=1
• Windows amd64 cross-compilation of internal/core
• independent code-quality and concurrency review: PASS

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.42%. Comparing base (94d233b) to head (6fd5b2f).

Files with missing lines	Patch %	Lines
internal/core/group_by_hnsw.go	80.00%	0 Missing and 1 partial ⚠️
internal/core/hnsw.go	95.83%	0 Missing and 1 partial ⚠️
internal/core/hnsw_parallel.go	88.88%	0 Missing and 1 partial ⚠️
internal/core/hnsw_quantized.go	94.44%	0 Missing and 1 partial ⚠️
internal/core/hnsw_sparse.go	95.83%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
+ Coverage   61.35%   61.42%   +0.06%     
==========================================
  Files         149      150       +1     
  Lines       40240    40285      +45     
==========================================
+ Hits        24689    24744      +55     
+ Misses      12340    12335       -5     
+ Partials     3211     3206       -5     

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