Remove MHD + Lax-Friedrichs

[ChrisZYJ]

Description

Bug fix, Closes #1661.

Deletes the unreachable MHD and RMHD branches from src/simulation/m_riemann_solver_lf.fpp (state blocks, momentum/energy flux arms, induction-equation flux block, c_fast computation, and the associated declarations/GPU-private-list entries). Adds the missing Fortran-side check in src/simulation/m_checker.fpp

Note: The deletion choice follows from the #1661 review's suggested resolution. The MHD code in LF was never reachable through the toolchain, and the implementation has subtle bugs (like Rusanov bound only using the acoustic sound speed).

Net LOC

src: −154/+16 (m_riemann_solver_lf.fpp +14/−154, m_checker.fpp +2).

Testing

Passed local tests.

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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[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.37%. Comparing base (ae4b4c4) to head (c7e5be3).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/simulation/m_riemann_solver_lf.fpp	50.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1722      +/-   ##
==========================================
+ Coverage   61.21%   61.37%   +0.16%     
==========================================
  Files          84       84              
  Lines       21601    21542      -59     
  Branches     3195     3189       -6     
==========================================
- Hits        13223    13222       -1     
+ Misses       6209     6153      -56     
+ Partials     2169     2167       -2     

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