Bound the burn sub-step's per-fluid locals by num_fluids_max

Under case optimization num_fluids is a compile-time constant, and the Case Opt lanes build cases that set it to one, where the literal reactant and product indices are then out of bounds and the compile fails. num_fluids_max is a parameter and always covers both, so the arrays no longer need the case-optimization and AMD split either.

Author: sbryngelson

src/simulation/m_reactive_burn.fpp

@@ -113,11 +113,10 @@ contains
         real(wp)                                               :: dt_sub, e_int, gamma_mix, pi_inf_mix, qv_mix
         real(wp)                                               :: rho_mix, dlambda, dmass
 
-        #:if not MFC_CASE_OPTIMIZATION and USING_AMD
-            real(wp), dimension(3) :: alpha_rho, alpha
-        #:else
-            real(wp), dimension(num_fluids) :: alpha_rho, alpha
-        #:endif
+        ! Bounded by num_fluids_max, not num_fluids: under case optimization num_fluids is a compile-time
+        ! constant that can be 1, and the reactant/product indices below are literal. The validator holds
+        ! reactive_burn to two fluids, so only the first two entries are ever used.
+        real(wp), dimension(num_fluids_max) :: alpha_rho, alpha
 
         dt_sub = dtime/real(rburn%substeps, wp)