Fix MHD hyperbolic cleaning using wrong sign for left fast magnetosonic speed

[sbryngelson]

Summary

Severity: HIGH — wrong wave speed estimate corrupts MHD flux computation.

File: src/simulation/m_riemann_solvers.fpp, lines 655-657

The hyperbolic divergence cleaning should ensure that the fast magnetosonic speed is at least the cleaning speed in both directions. The right state correctly uses max(c_fast%R, hyper_cleaning_speed), but the left state uses min(c_fast%L, -hyper_cleaning_speed), which forces it negative.

Before

if (hyper_cleaning) then
    c_fast%L = min(c_fast%L, -hyper_cleaning_speed)  ! forces negative
    c_fast%R = max(c_fast%R, hyper_cleaning_speed)    ! correct
end if

After

if (hyper_cleaning) then
    c_fast%L = max(c_fast%L, hyper_cleaning_speed)    ! symmetric with R
    c_fast%R = max(c_fast%R, hyper_cleaning_speed)
end if

Why this went undetected

MHD with hyperbolic cleaning is a specialized feature. The asymmetric wave speed produces subtly wrong fluxes that may not cause immediate crashes.

Test plan

• Run MHD simulation with hyperbolic cleaning enabled
• Verify wave speed estimates are symmetric between left and right states

🤖 Generated with Claude Code

Fixes #1201

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Pull request overview

Fixes the sign handling for left-state fast magnetosonic speed enforcement when hyperbolic cleaning is enabled, making it consistent with the right-state treatment.

Changes:

• Replace min(c_fast%L, -hyper_cleaning_speed) with max(c_fast%L, hyper_cleaning_speed) under hyperbolic cleaning.
• Keep right-state enforcement as max(c_fast%R, hyper_cleaning_speed) for symmetry.