fix: add IGR guard, shear-thinning mu_max check, and weno_Re_flux test

- Fortran checker: prohibit non-Newtonian + igr combination (IGR does not
  allocate primitive variable storage needed by HB viscosity)
- Fortran checker: require mu_max when nn < 1 (shear-thinning) to bound
  viscosity at zero shear rate
- case_validator.py: matching Python-side shear-thinning mu_max check
- cases.py: add weno_Re_flux=T variant to Non-Newtonian test block;
  generate golden files for 1D/2D/3D shear-thinning+weno_Re_flux

Author: sbryngelson

src/simulation/m_checker.fpp

@@ -109,12 +109,15 @@ contains
         do i = 1, num_fluids
             if (fluid_pp(i)%non_newtonian) then
                 @:PROHIBIT(.not. viscous, "Non-Newtonian fluid requires viscosity to be enabled")
+                @:PROHIBIT(igr, "Non-Newtonian viscosity is not supported with igr")
                 @:PROHIBIT(fluid_pp(i)%K <= 0._wp, "Non-Newtonian fluid consistency index K must be > 0")
                 @:PROHIBIT(fluid_pp(i)%nn <= 0._wp, "Non-Newtonian fluid flow behavior index nn must be > 0")
                 @:PROHIBIT(fluid_pp(i)%tau0 < 0._wp, "Non-Newtonian fluid yield stress tau0 must be >= 0")
                 @:PROHIBIT(fluid_pp(i)%mu_min < 0._wp, "Non-Newtonian fluid mu_min must be >= 0")
                 @:PROHIBIT(.not. f_is_default(fluid_pp(i)%mu_max) .and. fluid_pp(i)%mu_max <= fluid_pp(i)%mu_min, &
                            & "Non-Newtonian fluid mu_max must be > mu_min when set")
+                @:PROHIBIT(fluid_pp(i)%nn < 1._wp .and. f_is_default(fluid_pp(i)%mu_max), &
+                           & "Shear-thinning (nn < 1) requires mu_max to bound viscosity at zero shear rate")
                 @:PROHIBIT(fluid_pp(i)%hb_m <= 0._wp, "Non-Newtonian Papanastasiou parameter hb_m must be > 0")
             end if
         end do