MHD hyperbolic cleaning uses wrong sign for left fast magnetosonic speed #1201
Description
Bug
In `src/simulation/m_riemann_solvers.fpp`, the left wave speed for MHD hyperbolic cleaning uses `min(c_fast%L, -hyper_cleaning_speed)` which forces the speed negative. It should be `max(c_fast%L, hyper_cleaning_speed)` since the cleaning speed is a positive bound on the maximum signal speed.
File: `src/simulation/m_riemann_solvers.fpp`, line 656
Impact: The left wave speed is artificially clamped negative, making the HLL flux calculation incorrect for MHD simulations with hyperbolic divergence cleaning.
Severity: High — incorrect Riemann solver wave speeds in MHD simulations.