Add dense inverse and its tests

[Plusero]

Relates to #78

Selective inverse for block sparse matrices is a component for the Uncertainty Quantification of State Estimation. Dense inverse is a function needed by selective inverse for block sparse matrices.

Changes proposed in this PR

• Extract reusable forward and backward substitution logic from SparseLUSolver.
• Reuse the extracted substitution logic in DenseLUFactor.
• Add dense_inverse to DenseLUFactor.
• Add tests for dense_inverse, including reuse of an existing test matrix.
• Let dense LU factorization take lvalue ref, for better readability and removal of capturing into void.

[Plusero]

build-test-release / build-cpp-test-macos (release, macos-26-intel) fails in installing uv. It doesn't seem to originate from my commits. Should I do something on my side, or leave it as is?

[mgovers]

build-test-release / build-cpp-test-macos (release, macos-26-intel) fails in installing uv. It doesn't seem to originate from my commits. Should I do something on my side, or leave it as is?

looks like a connection issue. rerunning to double-check

[Plusero]

Clang-tidy failed. Will fix this afternoon.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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