Fix Eigen iterative solver lifetime problem

[iiiian]

Description

Eigen iterative solver stores a reference to input matrix A and expect it to remain valid.

See https://libeigen.gitlab.io/eigen/docs-nightly/classEigen_1_1IterativeSolverBase.html#a3b0d9691964793bd55937666426fefbf.

Currently we do not make a copy and this results in segmentation fault when A is destroyed before the compute() call. This PR fix this by manually copying A.

An alternative solution is to have a API with clear lifetime expectancy.

Changes

• Copy input matrix A in factorize().
• Unify input matrix naming: Most of the code base uses const StiffnessMatrix &A but some methods in Eigen solver use const StiffnessMatrix &K instead. I change the parameter name to A for consistency.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.67%. Comparing base (550f6a5) to head (662cacf).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #91   +/-   ##
=======================================
  Coverage   81.66%   81.67%           
=======================================
  Files          49       49           
  Lines        2024     2025    +1     
  Branches      269      269           
=======================================
+ Hits         1653     1654    +1     
  Misses        371      371           

Flag	Coverage Δ
polysolve	81.67% <100.00%> (+<0.01%)	⬆️

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