IGR integration

[StevenXue98]

Description

This PR integrates IGR with more of the shared RHS/Riemann infrastructure while preserving the existing IGR numerical behavior.

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 s_riemann_solver path using the LF Riemann solver.
• Use shared RHS accumulation helpers for IGR:
  • s_compute_advection_source_term
  • s_compute_additional_physics_rhs
• Refactor IGR-specific pieces out of the old fused s_igr_riemann_solver path:
  • s_igr_reconstruct_cell_boundary_values
  • s_igr_correct_lf_fluxes
  • s_igr_sigma
• Preserve legacy IGR behavior where needed:
  • skip the normal LF MPP limiter for IGR, since it was not part of the old IGR path
  • skip normal LF viscous-source construction for IGR, since IGR now fills those source-flux slots itself
• Remove the special padded rhs_vf allocation previously needed by IGR-specific direct RHS updates.

IGR-specific reconstruction, Sigma correction, and viscous source-flux construction are still present in m_igr.fpp. Subroutine s_igr_riemann_solver is also still present in m_igr.fpp, which explains the increased line count.

Type of change

• Refactor

Testing

Local testing:

./mfc.sh test --add-new-variables --only IGR
./mfc.sh test --add-new-variables --from F926448C --to F926448C
./mfc.sh test --add-new-variables --from 18A71AB2 --to 18A71AB2
./mfc.sh test --add-new-variables --from 43B5FEBD --to 43B5FEBD
./mfc.sh test --add-new-variables --from E49EF7B6 --to E49EF7B6
./mfc.sh test --only IGR

Additional local comparison/debug checks used during development:

python3 local_check_igr_full_alpha.py --default-igr
python3 local_igr_diagnostics/scripts/compare_igr_packs.py --default-igr

The IGR golden files were updated with --add-new-variables to include the newly stored final volume fraction. Existing golden entries were preserved, and the added final-alpha fields satisfy the expected volume-fraction sum checks, which is verified by the local Python scripts.

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
• Verified CPU only

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