Fix moncon_cutoff declared as integer, truncating 1e-8 to 0

moncon_cutoff is assigned 1e-8_wp but declared as integer, so Fortran
silently truncates it to 0. This makes all THINC monotonicity constraint
comparisons always true, completely disabling the constraint.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

src/common/m_constants.fpp

@@ -44,7 +44,7 @@ module m_constants
     ! Interface Compression
     real(wp), parameter :: dflt_ic_eps = 1e-4_wp !< Ensure compression is only applied to surface cells in THINC
     real(wp), parameter :: dflt_ic_beta = 1.6_wp !< Sharpness parameter's default value used in THINC
-    integer, parameter :: moncon_cutoff = 1e-8_wp !< Monotonicity constraint's limiter to prevent extremas in THINC
+    real(wp), parameter :: moncon_cutoff = 1e-8_wp !< Monotonicity constraint's limiter to prevent extremas in THINC
 
     ! Chemistry
     real(wp), parameter :: dflt_T_guess = 1200._wp ! Default guess for temperature (when a previous value is not available)