fix: prefetch native FP16 HNSW vectors

[zhangzhenghao]

Fixes #93.

Problem

Native VECTOR_FP16 HNSW indexes keep vectors in vectorsFP16, but query prefetch always read the FP32 vectors slice. Once graph traversal ran above the brute-force threshold, the empty FP32 slice caused an index-out-of-range panic.

Changes

• dispatch HNSW prefetch to the active FP16 or FP32 storage
• prefetch FP16 cache lines using 32 half-precision elements per 64-byte line
• use storage-aware prefetch in normal, block-heap, and group-search traversal
• add a collection-level regression covering the reported 1,682-vector, 64-dimensional cosine query

Verification

• regression test fails on origin/main with the reported panic and passes with this fix
• go test ./... -count=1
• go vet ./...
• git diff --check

Local golangci-lint could not run because the installed binary was built with Go 1.24 while this repository targets Go 1.27; GitHub CI uses its pinned compatible linter.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.39%. Comparing base (1a610c3) to head (5ea4bc6).

Files with missing lines	Patch %	Lines
internal/core/algorithm/hnsw_utility.go	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #94      +/-   ##
==========================================
- Coverage   81.64%   81.39%   -0.26%     
==========================================
  Files         150      150              
  Lines       30534    30551      +17     
==========================================
- Hits        24931    24867      -64     
- Misses       5601     5682      +81     
  Partials        2        2              

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