Relax Copy to Clone on Standard<T> matrix elements

[suri-kumkaran]

What

Mat<Standard<T>> required T: Copy. Now it works for any Clone element (and any T for views) — matching what diskann-utils::Matrix<T> already allows.

Why

The Copy bound wasn't load-bearing; it came from a naive derive and a few over-broad where clauses. Relaxing it is a prerequisite for unifying Matrix<T> with Mat<Standard<T>>.

How

• Standard<T>: hand-write unconditional Copy/Clone (it's just dims + PhantomData), drop the unused Eq, keep PartialEq.
• Drop T: Copy from Repr/ReprMut/ReprOwned/NewRef/NewMut and the Mat/MatRef/MatMut impl blocks.
• NewOwned/NewCloned: Copy → Clone (value.clone(), .cloned()).
• Add a String-element test covering fill / from_fn / mutation / clone / views.

Repr metadata stays Copy (required by Repr: Copy, free for PhantomData) — only the element bound relaxes.

[Copilot]

Pull request overview

This PR relaxes Mat<Standard<T>> to support non-Copy element types by switching construction/cloning paths from T: Copy to T: Clone, while keeping Standard<T> metadata Copy (so Repr: Copy continues to hold). This aligns multi_vector::Mat behavior more closely with diskann-utils::Matrix<T> and unblocks future unification work.

Changes:

• Relax T: Copy bounds to T: Clone (or remove entirely for views) across Standard<T>’s Repr*/New* impls and Mat/MatRef/MatMut impl blocks.
• Update owned initialization and cloning paths to use .clone() / .cloned() instead of copying.
• Add a String-backed test to validate fill, from_fn, mutation, clone, and slice-backed views for non-Copy elements.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.16%. Comparing base (1552e51) to head (00e516b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
diskann-quantization/src/multi_vector/matrix.rs	92.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1265      +/-   ##
==========================================
+ Coverage   90.16%   91.16%   +0.99%     
==========================================
  Files         509      510       +1     
  Lines       97238    97291      +53     
==========================================
+ Hits        87678    88694    +1016     
+ Misses       9560     8597     -963     

Flag	Coverage Δ
miri	91.16% <92.50%> (+0.99%)	⬆️
unittests	91.12% <92.50%> (+1.28%)	⬆️

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	95.53% <92.50%> (-0.18%)	⬇️

... and 53 files with indirect coverage changes

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[hildebrandmw]

Thanks! Just a few minor comments.