IGR integration

[StevenXue98]

Description

This draft PR begins integrating IGR with the shared RHS/Riemann infrastructure.

Main changes:

• Store the final volume fraction for IGR so the IGR state layout matches the normal model-equations layout more closely.
• Route IGR face states through the shared LF Riemann solver path.
• Use the shared advection-source RHS accumulation path for IGR.
• Split remaining IGR-specific Sigma and viscous RHS logic into separate helper routines.
• Keep legacy IGR numerics where needed, including an IGR-gated LF MPP limiter behavior to preserve previous IGR results.

This is intended as a first integration step. IGR-specific reconstruction, Sigma correction, and viscous correction logic are still present in m_igr.fpp.

Type of change

• Refactor

Testing

Tested with local IGR regression comparison:

./mfc.sh test --only IGR
./mfc.sh test -j 1 --debug --from F926448C --to F926448C
./mfc.sh test -j 1 --debug --from 18A71AB2 --to 18A71AB2
./mfc.sh test -j 1 --debug --from 43B5FEBD --to 43B5FEBD
./mfc.sh test -j 1 --debug --from E49EF7B6 --to E49EF7B6
python3 local_check_igr_full_alpha.py --default-igr
python3 local_igr_diagnostics/scripts/compare_igr_packs.py --default-igr

All new outputs match the golden files to machine precision except for case E49EF7B6. E49EF7B6 uses a looser existing tolerance, so I also compared against a regenerated upstream/reference pack.txt. That comparison passed with max relative difference around 2.6e-11.

Checklist

Check these like this [x] to indicate which of the below applies.

• I added or updated tests for new behavior
• I updated documentation if user-facing behavior changed

See the developer guide for full coding standards.

GPU changes (expand if you modified src/simulation/)

• GPU results match CPU results
• Tested on NVIDIA GPU or AMD GPU

AI code reviews

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[StevenXue98]

Small status note for this draft PR:

This is still a work in progress. The current version is intended to show the first IGR integration step: routing IGR through the shared LF Riemann solver (now calling s_riemann_solver) and shared advection RHS accumulation (now calling s_compute_advection_source_term) while preserving legacy IGR results.

A few things are still intentionally unfinished:

• The line count is currently larger because some legacy IGR logic is still present while the new path is being validated.
• IGR-specific reconstruction, Sigma correction, and viscous correction are still in m_igr.fpp.
• The next cleanup target is to reduce duplicated IGR viscous/reconstruction logic and move more RHS accumulation through existing shared helpers (e.g. s_compute_additional_physics_rhs, and later potentially s_get_viscous, which requires changing the custom IGR logic).
• Some official test failures are expected due to the expanded final-volume-fraction output/golden-file line count mismatch rather than numerical mismatch. The IGR-specific test cases are currently verified through a local comparison script while we decide how to handle goldens.

I’ll keep pushing smaller cleanup/refactor commits as each piece is validated.