refactor(core): align layout with zvec

[zhangzhenghao]

Summary

• move the cohesive vector index implementation into internal/core/algorithm
• rename implementation and test files to follow Alibaba zvec src/core terminology
• update internal consumers and examples to use the new package path
• keep internal/core/metric as the existing independent leaf package

The implementation remains a single Go package because DiskANN, Vamana, HNSW variants, IVF, quantizers, and query utilities currently share private representations and helpers. This keeps the change behavior-preserving without exposing implementation details solely to force package boundaries.

Verification

• go list ./internal/core/...
• go test ./internal/core/...
• go test ./...
• go test -race ./...
• go vet ./...
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.79%. Comparing base (17cad6e) to head (8b64616).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   82.78%   82.79%   +0.01%     
==========================================
  Files         118      118              
  Lines       25089    25089              
==========================================
+ Hits        20769    20773       +4     
+ Misses       4320     4316       -4     

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