Use Native SHOC cloud fraction with RRTMGP

[wang1202]

Summary

Use Native SHOC's diagnosed liquid-cloud fraction in the RRTMGP radiation calculation when Native SHOC and RRTMGP are enabled together.

This preserves the legacy binary cloud-fraction behavior for non-Native-SHOC configurations and provides a runtime switch for controlled comparison runs.

Implementation

• Added erf.rad_use_shoc_cldfrac, defaulting to true.
• Added an optional liquid-cloud-fraction field to the radiation interface.
• Reordered Native SHOC + RRTMGP radiation so SHOC runs first and radiation receives the current-step diagnostic and updated atmospheric state.
• Added separate liquid, ice, and total cloud fractions:
  • liquid fraction uses Native SHOC when available and enabled;
  • ice fraction remains binary;
  • total fraction is max(liquid fraction, ice fraction).
• Liquid water path now uses the liquid fraction, while ice water path uses the binary ice fraction.
• Added validation for the optional cloud-fraction MultiFab layout.
• Added focused unit tests for fraction selection, bounds, and ice-only clouds.
• Updated the input documentation and Native SHOC README.

Scope and limitations

• Cloud ice retains a binary cloud fraction because Native SHOC currently diagnoses liquid-cloud macrophysics only.
• Snow, graupel, effective-radius, cloud-overlap, and EAMxx SHOC coupling were not changed.