Adding subview_mut to MatrixMut

[Aditya Krishnan (arkrishn94)]

This PR introduces a subview_mut method to the recently added MatrixMut trait so that we can copy data in bulk into "subviews" of a mutable matrix.

So copying data from a ref: Ref<'_, T> into a mut: Mut<'_, T> can be done with something like this -

let range = 0..ref.nrows();
let subview = mut.subview_mut(range).ok_or(panic!("incorrect range")); 
subview.as_mut_slice().copy_from_slice(ref.as_slice())

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation preserves existing layout and borrowing contracts, with tests covering range validation and nested writes.

Review effort: Balanced
Findings: None

What changed in this PR

Adds mutable row subviews to MatrixMut, enabling bulk writes into selected matrix rows.

Changes:

• Adds subview_mut with range validation.
• Extends tests for boundary cases, zero-column matrices, and nested mutations.

File	Description
diskann-utils/​src/​views/​rowmajor.rs	Implements mutable subviews and extends subview tests.

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[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.92%. Comparing base (bab8136) to head (f33ae73).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1463   +/-   ##
=======================================
  Coverage   91.92%   91.92%           
=======================================
  Files         583      583           
  Lines      115876   115903   +27     
=======================================
+ Hits       106523   106549   +26     
- Misses       9353     9354    +1     

Flag	Coverage Δ
miri	91.92% <100.00%> (+<0.01%)	⬆️
unittests	91.87% <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-utils/src/views/rowmajor.rs	96.96% <100.00%> (+0.06%)	⬆️

... and 6 files with indirect coverage changes

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