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

[sbryngelson]

Bug

In `src/common/m_constants.fpp`, `moncon_cutoff` is declared as `integer, parameter` but assigned the value `1e-8_wp`. Fortran silently truncates the real to integer, setting `moncon_cutoff = 0`.

File: `src/common/m_constants.fpp`

Impact: The monotonicity check cutoff is 0 instead of 1e-8, making the check `abs(delta) < 0` which is always false. The monotonicity constraint is effectively disabled.

Severity: Critical — monotonicity limiting is silently bypassed.

[sbryngelson]

Fix: #1176