Fix integral zmin/zmax never initialized (copy-paste of ymin/ymax)

The z-bounds initialization for volume integrals repeats ymin/ymax
instead of zmin/zmax. 3D volume integrals use uninitialized z-bounds.

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

Author: sbryngelson

src/simulation/m_global_parameters.fpp

@@ -814,8 +814,8 @@ contains
             integral(i)%xmax = dflt_real
             integral(i)%ymin = dflt_real
             integral(i)%ymax = dflt_real
-            integral(i)%ymin = dflt_real
-            integral(i)%ymax = dflt_real
+            integral(i)%zmin = dflt_real
+            integral(i)%zmax = dflt_real
         end do
 
         ! GRCBC flags