Diag: Clear Air Turbulence

This ticket is a directive for Claude:

Migrate the UM diagnostic "Clear Air Turbulence" to LFRic.
The UM code can be found in atmosphere/PWS_diagnostics/pws_cat_mod.F90.

Edit: also max cat and max cat pressure.

Start by reading atmosphere/PWS_diagnostics/pws_cat_mod.F90 to understand the UM implementations of Clear Air Turbulence, maximum CAT, and maximum CAT pressure. Trace how the corresponding diagnostic work is organized in LFRic, then confirm completion by migrating all three diagnostics and checking the relevant diagnostic tests or validation workflow identified during the investigation.

Written by the indexing model from the issue text.