Operator-split integration for the condensed-phase reactive burn

[sbryngelson]

Added to the flow RHS, the condensed-phase burn ties the reaction time scale to the acoustic CFL: a fast burn forces a smaller step for the entire simulation, even where nothing is reacting.

This adds rburn%substeps. Above zero, the flow is frozen and the burn ODE is integrated across the step in equal sub-steps, with the mixture pressure re-evaluated from the frozen internal energy each sub-step so the rate feels its own feedback as reactant becomes product. rburn%substeps = 0 is the default and leaves the source in the flow RHS exactly as before.

Two details worth review:

• Completion. A sub-step longer than the reaction time would carry the progress variable past one. It stops at one and hands over the reactant's remaining mass in the same sub-step. Capping the volume-fraction and mass increments independently strands reactant mass at zero reactant volume, and the mixture EOS divides one by the other.
• One rate law. s_burn_rate now states the pressure drive, the (1 - lambda) factor and the optional Arrhenius term once; both integrators call it. Previously the new path would have carried a second copy that had to stay in step forever.

Testing

• 1D -> Reactive Burn -> Condensed Programmed Detonation -> substeps (86893F55), substeps = 10 on 2 ranks. Two ranks deliberately: substeps is the only integer among the rburn members, and the broadcast kind is now registry-driven, so a wrong kind leaves rank 1 sub-stepping a garbage count — invisible to a single-rank golden.
• The new golden differs from the RHS-path golden in 45.5% of values, largest relative difference 3.2e-3 in the progress variable, so the case exercises the split integrator rather than reproducing the old answer.
• The four existing reactive-burn goldens pass unchanged on both CPU and GPU, confirming the rate-law extraction altered no behaviour.

[Copilot]

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Pull request overview

Adds an operator-split path for condensed-phase reactive burn by introducing rburn%substeps, allowing the burn ODE to be integrated in post-flow substeps so fast chemistry doesn’t globally constrain the acoustic CFL time step.

Changes:

• Register/validate new parameter rburn%substeps and ensure its MPI broadcast uses the correct MPI datatype (integer vs real).
• Integrate reactive burn via operator splitting after the flow update when rburn%substeps > 0, leaving the existing RHS source path for substeps == 0.
• Add a new 2-rank golden test case to exercise the new substepping path and catch broadcast-kind issues.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
toolchain/mfc/test/cases.py	Adds a new reactive-burn test case with rburn%substeps=10 on 2 MPI ranks.
toolchain/mfc/params/generators/fortran_gen.py	Updates generated MPI_BCAST code to pick MPI type per rburn member (supports new INT member).
toolchain/mfc/params/definitions.py	Registers rburn%substeps as an INT reactive-burn parameter.
toolchain/mfc/case_validator.py	Validates rburn%substeps >= 0 with an explanatory error message.
src/simulation/m_time_steppers.fpp	Calls s_reactive_burn_substep after the flow update when substeps > 0.
src/simulation/m_rhs.fpp	Skips adding burn source to RHS when substeps > 0 (operator-split path instead).
src/simulation/m_reactive_burn.fpp	Factors out s_burn_rate and implements operator-split substep integrator.
src/common/m_global_parameters_common.fpp	Initializes rburn%substeps default to 0.
src/common/m_derived_types.fpp	Extends reactive burn parameter type with integer substeps.
tests/86893F55/golden.txt	Adds new golden outputs for the new substepping test case.
tests/86893F55/golden-metadata.txt	Adds metadata for the new golden generation.

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[sbryngelson]

The second alternative already holds: case_validator.py:1932 enforces reactive_burn requires num_fluids = 2 (reactant then product), so fluids 1 and 2 are the whole mixture here. s_compute_reactive_burn computes rho the same way for the same reason. Leaving as is.

[codecov]

Codecov Report

❌ Patch coverage is 84.44444% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.35%. Comparing base (70589fd) to head (7b42ba5).

Files with missing lines	Patch %	Lines
src/simulation/m_reactive_burn.fpp	82.05%	2 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1831      +/-   ##
==========================================
+ Coverage   62.30%   62.35%   +0.04%     
==========================================
  Files          84       84              
  Lines       21583    21621      +38     
  Branches     3195     3199       +4     
==========================================
+ Hits        13448    13482      +34     
- Misses       5937     5939       +2     
- Partials     2198     2200       +2     

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[github-actions]

Lines of Code

File	Lines	Diff
src/simulation/m_reactive_burn.fpp	107	+66
src/simulation/m_time_steppers.fpp	833	+6
src/common/m_derived_types.fpp	441	+1
src/common/m_global_parameters_common.fpp	240	+1

Directory	Lines	Diff
common	9948	+2
simulation	27740	+72
total	45545	+74