Add function to remove boundary contributions

[greole]

This PR adds a function to remove boundary contributions from the matrix diagonal. The reasoning is that other CFD frameworks might or might not add boundary contributions to the diagonal matrix. Currently, NeoN directly adds the boundary contributions to the diagonal matrix and stores them in the boundMatrix. To simplify testing a function is added to remove these contributions.

Additionally:

• This PR fixes the inconsistent handling of the boundary RHS vector where the values are overwritten instead of added by the operator.
• Not installing all required fmt targets
• add .bck and .claude to gitignore. This also prevents reuse from complaining

[github-actions]

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[Copilot]

Pull request overview

This PR aims to adjust how boundary contributions are tracked/applied in finite-volume operator assembly (and adds a helper to remove those contributions for testing), alongside a new lower() matrix utility declaration.

Changes:

• Update boundary-face accumulation behavior in Gauss-Green Laplacian/Div implicit assembly (boundary matrix/RHS tracking and diagonal updates).
• Add removeBoundaryContributions(...) helper on LinearSystem for test workflows.
• Declare a new lower(...) triangular extraction function in the linear algebra API.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
src/linearAlgebra/matrix.cpp	Minor formatting-only change (extra blank line).
src/finiteVolume/cellCentred/operators/gaussGreenLaplacian.cpp	Changes boundary-face diagonal/RHS updates and boundary tracking.
src/finiteVolume/cellCentred/operators/gaussGreenDiv.cpp	Changes boundary-face diagonal/RHS updates and boundary tracking (implicit path).
include/NeoN/linearAlgebra/matrix.hpp	Declares lower(...) and adds Doxygen comments.
include/NeoN/linearAlgebra/linearSystem.hpp	Adds removeBoundaryContributions(...) testing helper.

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[Copilot]

This diff introduces an extra blank line between functions. If the project is keeping spacing tight in this file, consider removing it to avoid noisy diffs.

[chihtaw]

Is the function for removing boundary contributions solely for the purpose of testing in distributed settings?

[greole]

As of now, solely for testing purposes.

[chihtaw]

We need proper unit tests to validate the boundary matrix and boundary RHS in operator evaluations so that bugs won't easily sneak into the operator evaluations next time.

[chihtaw]

It looks like the unit test added by this PR (exasim-project/NeoFOAM#275) only validates coefficient matrix of linear system of equations for pressure-velocity coupling on NeoFOAM side.

I think we need unit tests on NeoN side to validate the RHS of linear system of equations without pressure-velocity coupling which only happens on NeoFOAM.

[chihtaw]

LGTM!
Just a comment on the documentation for removeBoundaryContributions.