[multi-vector] Enable cloning of Mat and friends.

[hildebrandmw]

Enable:

1. Clone for Mat
2. MatRef::to_owned (as an inherent method)
3. MatMut::to_owned (as an inherent method)

All three of these require the underlying representation to implement NewCloned to perform the actual clone. This PR implements NewCloned for the Standard representation.

This is turning out necessary for use in multi-vector scenarios.

[Copilot]

Pull request overview

This PR enables cloning functionality for the multi-vector matrix types in the quantization module. It introduces a new NewCloned trait that representations can implement to support creating owned Mat instances from borrowed views. The PR implements this trait for the Standard<T> representation and uses it to enable Clone for Mat<T>, as well as adding to_owned() methods for MatRef and MatMut.

Changes:

• Introduced NewCloned trait for representations that support cloning from a view
• Implemented NewCloned for Standard<T> where T: Copy
• Added Clone implementation for Mat<T> where T: NewCloned
• Added to_owned() inherent methods to MatRef and MatMut
• Refactored NewOwned implementation to use a new box_to_mat helper method
• Added comprehensive tests for all three cloning paths

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.00%. Comparing base (26fd1af) to head (89f0514).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #759      +/-   ##
==========================================
+ Coverage   88.99%   89.00%   +0.01%     
==========================================
  Files         428      428              
  Lines       78291    78346      +55     
==========================================
+ Hits        69677    69734      +57     
+ Misses       8614     8612       -2     

Flag	Coverage Δ
miri	89.00% <100.00%> (+0.01%)	⬆️
unittests	89.00% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
diskann-quantization/src/multi_vector/matrix.rs	97.53% <100.00%> (+0.22%)	⬆️

... and 1 file with indirect coverage changes

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