perf: accelerate distance kernels with SIMD

[zhangzhenghao]

Summary

• add an internal/floats package with pure-float32 scalar, AVX, AVX-512, and NEON kernels for squared L2, inner product, and one-pass dot/norm accumulation
• generate architecture implementations with GoAT, dispatch by runtime CPU features, and retain a noasm scalar fallback
• route checked and prevalidated dense metrics through the new kernels while preserving validation and zero-vector behavior
• skip architecture-specific tests when the current CPU lacks the required instruction set

Performance

Measured on an Intel Core i7-4500U with AVX, comparing default dispatch against -tags=noasm:

Kernel (1536 dimensions)	noasm	AVX
L2 squared	1598 ns/op	210.6 ns/op
Inner product	1597 ns/op	208.1 ns/op
Dot + norms	1914 ns/op	242.0 ns/op

All benchmarks report 0 B/op and 0 allocs/op.

Verification

• go test ./... -count=1
• go test -tags=noasm ./... -count=1
• go vet ./...
• go test ./internal/floats ./internal/ailego -count=10
• GoAT regeneration hash comparison
• Linux ARM64, Darwin ARM64, Windows AMD64, and Windows ARM64 test-binary cross-compilation
• host AVX-specific tests pass; AVX-512-specific test skips because the host CPU does not support AVX-512

Local golangci-lint was not used because the installed binary was built with Go 1.24 and refuses modules targeting Go 1.26; the repository CI uses its pinned Go 1.26-compatible lint action.

[codecov]

Codecov Report

❌ Patch coverage is 77.14286% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.32%. Comparing base (579bc98) to head (f8a975d).

Files with missing lines	Patch %	Lines
internal/floats/floats_amd64.go	52.00%	24 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
- Coverage   61.33%   61.32%   -0.02%     
==========================================
  Files         145      147       +2     
  Lines       40173    40203      +30     
==========================================
+ Hits        24642    24654      +12     
- Misses      12318    12339      +21     
+ Partials     3213     3210       -3     

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