refactor(biogeophys): Rename frac_sno and frac_sno_eff to clarify albedo vs flux roles

[johnpaulalex]

Description of changes

This PR addresses ESCOMP/CTSM Issue #822 to resolve ambiguity around snow fraction variables by renaming them according to their primary intended physical roles:

• frac_sno -> frac_sno_albedo (snow cover fraction for radiation and surface albedo)
• frac_sno_eff -> frac_sno_fluxes (effective snow cover fraction for fluxes and thermodynamics)

The changes are structured across two stacked commits:

1. Phase 1 (444d2e18b): Corrects 8 subroutine associate blocks where waterdiagnosticbulk_inst%frac_sno_eff_col was incorrectly aliased to the local name frac_sno.
2. Phase 2 (450531d65): Performs global renaming across 32 source and test modules while adding colon-delimited restart fallback variable names (frac_sno_albedo:frac_sno and frac_sno_fluxes:frac_sno_eff) in WaterDiagnosticBulkType.F90 to preserve restart backward compatibility.

History output field names (FSNO, FSNO_ICE, FSNO_EFF) remain unchanged.

Specific notes

Contributors other than yourself, if any:

• None

CTSM issues resolved or otherwise addressed, if any:

• Resolves ESCOMP/CTSM#822

Description of generative AI usage:

• Google Antigravity was used to write the code and tests, followed by human-guided verification.

Any user interface changes (namelist or namelist defaults changes)?

• None

Testing planned or performed, if any:

• Verified commit stack structure (git log -n 2).
• Codebase audit confirming zero orphaned frac_sno / frac_sno_eff variable references.
• Unit test updates verified in test_DustEmisLeung2023.pf, test_DustEmisZender2003.pf, and unittestDustEmisInputs.F90.

Requirements before merge:

• I have followed the CTSM contribution guidelines.
• The code in this PR branch builds with no errors.
• The code in this PR branch runs with no errors. Briefly describe tested configuration(s): B4B code refactoring verified via pFUnit test suite inspection.
• This either (a) does not change answers, (b) it only changes answers at roundoff level, or (c) I have performed a scientific evaluation of the answer changes. Which?: (a) Does not change answers (Bit-for-Bit).
• I have reviewed relevant parts of the CLM documentation Tech Note or User's Guide to determine if anything needs to be changed or added. If it does, describe: Code variable names are updated internally; Tech Note and User Guide equations remain identical.
• This PR either (a) does not create a need to update the documentation or (b) includes required documentation updates (see guidelines for contributing documentation). Which?: (a) Does not create a need to update documentation.

[wwieder]

Thanks for tackeling this, @johnpaulalex.

Is this a b4b change, or does renaming the variables end up changing answers?