Use SciPy LAPACK to simplify Rust dependency stack

[mdekstrand]

This removes nalgebra, instead using the sposv LAPACK function through SciPy, simplifying the Rust dependency stack.

It also adds a general function for extracting Cython CABI function pointers that can be used to access additional BLAS and LAPACK functions in the future, if needed.

[codecov]

Codecov Report

❌ Patch coverage is 85.29412% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.90%. Comparing base (f3455da) to head (902d09f).
⚠️ Report is 11 commits behind head on future/2026.

Files with missing lines	Patch %	Lines
src/accel/als/solve.rs	79.16%	10 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           future/2026     #960      +/-   ##
===============================================
- Coverage        88.94%   88.90%   -0.04%     
===============================================
  Files              215      217       +2     
  Lines            14961    15012      +51     
===============================================
+ Hits             13307    13347      +40     
- Misses            1654     1665      +11     

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