Silent blow-up when riemann_solver is unspecified: default dflt_int=-100 dispatches no solver, flux_n retains garbage

[sbryngelson]

Summary

When a case file does not set riemann_solver, the Fortran variable is initialized to dflt_int = -100 (defined in m_global_parameters.fpp). The Fypp dispatch loop in m_riemann_solvers.fpp only handles solvers 1 (HLL), 2 (HLLC), 4 (HLLD), and 5 (LF):

#:for NAME, NUM in [('hll', 1), ('hllc', 2), ('hlld', 4), ('lf', 5)]
    if (riemann_solver == ${NUM}$) then
        call s_${NAME}$_riemann_solver(...)
    end if
#:endfor

With riemann_solver = -100, none of these branches execute. s_initialize_riemann_solver does not zero flux_n (it only conditionally zeros flux_src_vf for viscous/surface tension cases). In a release build (no -finit-real=snan), the flux_n allocatable array retains whatever uninitialized memory the allocator returns. On the very first RHS evaluation the flux divergence uses this garbage, causing immediate catastrophic blow-up (density → 10²⁶¹ after one step).

Reproduction

Add a 2D case that omits riemann_solver from the case dict. The simulation will blow up on step 1 with no diagnostic message.

Root cause

Three separate issues compound:

1. No solver dispatch for dflt_int: riemann_solver = -100 is not in [1, 2, 4, 5], so flux_n is never written.
2. flux_n not zeroed at startup: s_initialize_riemann_solver zeroes flux_src_vf only under viscous .or. surface_tension, never flux_n itself.
3. Case validator silently skips the check: toolchain/mfc/case_validator.py line 663 has if riemann_solver is None: return, allowing the missing parameter to pass validation without a warning.

Impact

Any case file that forgets riemann_solver (including all cases using the default) blows up silently on step 1 with no useful error message. This was the root cause of a blow-up discovered while developing the 2D axisymmetric convergence test.

Suggested fixes

• Short-term: zero flux_n in s_initialize_riemann_solver (safe, small cost), or add a @:PROHIBIT(riemann_solver == dflt_int, "riemann_solver must be specified") check at startup.
• Longer-term: give riemann_solver a meaningful default (e.g. 2 for HLLC) in m_global_parameters.fpp, and remove the silent return in case_validator.py.