refactor(core): extract metric package

[zhangzhenghao]

Summary

• extract dense-vector metric computation and ordering into internal/core/metric
• preserve existing core.Metric and metric constants through aliases while establishing the lower-level dependency boundary
• expose metric validity and prevalidated kernel selection for later algorithm package extraction
• move metric-specific tests to the new package

This is the first small step of restructuring internal/core after Alibaba zvec’s core/metric, core/quantizer, and core/algorithm responsibility split. It intentionally makes no scoring, ordering, persistence, or public API behavior changes.

Verification

• go test ./...
• go test -race ./...
• go vet ./...
• git diff --check
• go test ./internal/core/metric -coverprofile=/tmp/xvec-core-metric.cover (100% statement coverage)

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.39%. Comparing base (236f506) to head (2a80530).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/core/quantized_index.go	0.00%	0 Missing and 1 partial ⚠️
internal/core/refiner.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #25   +/-   ##
=======================================
  Coverage   61.38%   61.39%           
=======================================
  Files         155      156    +1     
  Lines       40415    40415           
=======================================
+ Hits        24810    24814    +4     
+ Misses      12367    12365    -2     
+ Partials     3238     3236    -2     

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[zhangzhenghao]

Closing this PR because the verified commit will be pushed directly to main.