feat(math): complete SIMD distance kernels

[zhangzhenghao]

Summary

• add AVX-512 FP32 batch dot-product and squared-L2 kernels
• add dedicated FP16 batch kernels for AVX-512, NEON, and LASX
• add LASX FP16 single-vector L2, dot-product, and fused dot/norm kernels
• consolidate math_batch FP16 scalar APIs and tests into distance.go and distance_test.go
• keep RVV FP16 on the scalar path because the current CPU feature source exposes base V but not the optional Zvfhmin extension required for safe dispatch

Verification

• go test -p 1 ./...
• go test -p 1 -tags noasm ./...
• go vet ./...
• make test-neon in internal/ailego/math_batch
• make test-lasx in internal/ailego/math and internal/ailego/math_batch
• make test-rvv in internal/ailego/math and internal/ailego/math_batch
• regenerated affected GoAT outputs twice and verified identical SHA-256 hashes
• inspected generated assembly for AVX-512 ZMM, NEON FP16, and LASX FP16 instructions

Notes

The architecture-specific tests skip when the required CPU features are unavailable. FP16 NEON dispatch requires both HasFPHP and HasASIMDHP.

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[codecov]

Codecov Report

❌ Patch coverage is 24.17582% with 69 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.23%. Comparing base (6c235ed) to head (10ca6ed).

Files with missing lines	Patch %	Lines
internal/ailego/math_batch/distance_amd64.go	2.70%	36 Missing ⚠️
internal/ailego/math_batch/distance_fp16_amd64.go	2.94%	33 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #96      +/-   ##
==========================================
- Coverage   81.41%   81.23%   -0.18%     
==========================================
  Files         150      149       -1     
  Lines       30672    30743      +71     
==========================================
+ Hits        24971    24974       +3     
- Misses       5699     5767      +68     
  Partials        2        2              

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