Fix FSILS RCS diagonal cancellation and general block scaling

[zasexton]

Current situation

Fixes #646.

The FSILS row and column scaling (RCS) preconditioner evaluates (a - 1.0) + 1.0 for unconstrained matrix diagonals while applying Dirichlet conditions. In double precision, this perturbs small coefficients and can erase them before scaling begins. The resulting operator no longer represents the assembled system, which can degrade convergence and solution accuracy across physics using this shared preconditioner.

For block sizes greater than four, column scaling also skips the last row of each block, and the row-maximum calculation omits the first entry. The latter can produce infinite scaling weights when a constrained diagonal is the only nonzero entry in its row.

Release Notes

• Preserve unconstrained RCS diagonals and set only constrained diagonals to one, using the normalized boundary mask for every block size.
• Correct the general block path to scale every block row and include every entry when computing row maxima.
• Add RCS matrix regression coverage for small coefficients, boundary constraints, and block sizes one through five.

Documentation

The numerical cancellation guard is documented inline in Code/Source/linear_solver/precond.cpp. The regression test documents its sparse matrix stencil and scale-normalized error checks.

Testing

• 327 unit tests passed, including 75 RCS matrix cases. These cover block sizes 1–5, coefficient scales 1, 1e-11, 1e-15, 1e-17, and 1e-20, and excluded, partially constrained, and fully constrained boundary faces.
• The matrix tests verify finite scaling weights, the reconstructed operator and right-hand side, the recovered solution, and the residual against the original system. Error checks are normalized by coefficient scale so lost small entries cannot pass through an absolute tolerance.
• Before their respective fixes, 48 small-coefficient cases reproduced diagonal corruption, including complete erasure, and 15 five-component cases reproduced the general block defects. All 75 cases pass with the fixes.
• 15 existing physics regressions passed on 1, 3, and 4 MPI ranks: heat diffusion with CG, BICG, and GMRES; CEP spiral_BO_2d; and the Stokes P1P1/N004 manufactured solution.
• Three additional Stokes comparisons passed, using temporary configurations that explicitly select RCS on 1, 3, and 4 MPI ranks.

Code of Conduct & Contributing Guidelines

• I agree to follow the Code of Conduct and Contributing Guidelines.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[zasexton]

adding @kko27 and @michelebucelli to review this PR since RCS preconditioning is occasionally used with CEP physics. Mostly want to know if we should add the unit test for regression assessments or instead remove that file and just have the bug fix only in this PR.

[kko27]

Thanks for fixing this!

[codecov]

Codecov Report

❌ Patch coverage is 98.91304% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.41%. Comparing base (c3f0bb8) to head (2d59b06).

Files with missing lines	Patch %	Lines
...unitTests/linear_solver_tests/test_precond_rcs.cpp	98.82%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #647      +/-   ##
==========================================
+ Coverage   73.08%   73.41%   +0.32%     
==========================================
  Files         268      269       +1     
  Lines       40270    40337      +67     
  Branches     6738     6755      +17     
==========================================
+ Hits        29433    29612     +179     
+ Misses      10594    10482     -112     
  Partials      243      243              

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[michelebucelli]

@zasexton I have no comments besides the one about detailing the test rationale a bit more.