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

[sbryngelson]

Bug

In `src/simulation/m_global_parameters.fpp`, the integral region z-bounds initialization copies the y-bounds instead of the z-bounds:

```fortran
integral(i)%ymin = ... ! should be integral(i)%zmin
integral(i)%ymax = ... ! should be integral(i)%zmax
```

File: `src/simulation/m_global_parameters.fpp`, lines 817-818

Impact: Integral regions in 3D simulations have uninitialized z-bounds, causing incorrect volume integration bounds. The y-bounds are written twice while z-bounds retain default values.

Severity: Critical — integral region z-bounds are garbage in 3D.

[sbryngelson]

Fix: #1175