Initialize k_v and k_g module-level variables alongside bub_pp counterparts

The bub_pp%k_v and bub_pp%k_g struct fields were initialized to dflt_real
but the corresponding module-level k_v and k_g scalars were not. Fixes
potential uninitialized reads when the thermal bubble model is inactive.

Fixes #1203

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

Author: sbryngelson

src/simulation/m_global_parameters.fpp

@@ -700,8 +700,8 @@ contains
         bub_pp%gam_g = dflt_real; gam_g = dflt_real
         bub_pp%M_v = dflt_real; M_v = dflt_real
         bub_pp%M_g = dflt_real; M_g = dflt_real
-        bub_pp%k_v = dflt_real; 
-        bub_pp%k_g = dflt_real; 
+        bub_pp%k_v = dflt_real; k_v = dflt_real
+        bub_pp%k_g = dflt_real; k_g = dflt_real
         bub_pp%cp_v = dflt_real; cp_v = dflt_real
         bub_pp%cp_g = dflt_real; cp_g = dflt_real
         bub_pp%R_v = dflt_real; R_v = dflt_real