[quantization] Add as_raw_ptr.

[Mark Hildebrand (hildebrandmw)]

Add publicly exported as_raw_ptr for retrieving the base pointer of Mat, MatRef, and MatMut.

The term "raw" is meant to denote that this is the byte pointer - not necessarily the pointer to the actual element type stored in these containers.

[Copilot]

Pull request overview

This pull request adds a new public method as_raw_ptr() to the Mat, MatRef, and MatMut types in the quantization module. The method provides safe access to the underlying byte-level pointer (*const u8) for these matrix types. The "raw" naming convention clearly distinguishes this byte-pointer accessor from potential typed-pointer accessors, following established codebase patterns.

Changes:

• Added as_raw_ptr() method to Mat, MatRef, and MatMut that returns the base byte pointer (*const u8)
• Removed the previous test-only as_ptr() method from Mat and replaced all its usages with the new public as_raw_ptr()
• Updated all test code to use the new method name

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[Codecov Comments Bot (codecov-commenter)]

Codecov Report

❌ Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.99%. Comparing base (e093583) to head (a84331d).

Files with missing lines	Patch %	Lines
diskann-quantization/src/multi_vector/matrix.rs	91.30%	2 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #774 +/- ##
==========================================
- Coverage 88.99% 88.99% -0.01% 
==========================================
Files 428 428 Lines 78415 78431 +16 ==========================================
+ Hits 69784 69798 +14 - Misses 8631 8633 +2 

Flag	Coverage Δ
miri	88.99% <91.30%> (-0.01%)	⬇️
unittests	88.99% <91.30%> (-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	97.28% <91.30%> (-0.25%)	⬇️

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