refactor(hydraulics): implement MatSolve2D_explicit kernel and pFUnit test suite

[johnpaulalex]

Description:

Refactors primitive plant hydraulics calculation routines (UpdatePlantPsiFTCFromTheta_explicit, UpdatePlantKmax_explicit, SumBetweenDepths, MatSolve2D_explicit) in biogeophys/FatesPlantHydraulicsMod.F90 to accept explicit parameter signatures rather than relying on global model singletons. Adds pFUnit test suite fortification for shallow partial layer integration (i_rhiz_b == 0) on non-uniform vertical soil grids, exact series resistance conservation, and multi-tier solver verification (smoke tests, extreme boundary cases, and mass conservation sweeps).

Specific notes

Collaborators:

• @jpalex

Linked issues addressed, if any:

• Contributes to NGEET/fates#1409

Expectation of Answer Changes:

• Bit-for-Bit (B4B) with baseline master
• Roundoff-level differences only
• Expected Answer Changes (ECA)

Description of generative AI usage (as necessary)

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

Checklist

Contributor

• The in-code documentation has been updated with descriptive comments
• The documentation has been assessed to determine if updates are necessary
• Describe use of generative AI (if necessary)

Integrator

• FATES PASS/FAIL regression tests were run
• Evaluation of test results for answer changes was performed and results provided
• FATES-CLM6 Code Freeze: satellite phenology regression tests are b4b

Test Results:

CTSM (or) E3SM (specify which) test hash-tag: N/A
CTSM (or) E3SM (specify which) baseline hash-tag: N/A
FATES baseline hash-tag: 081aca6b

Test Output:
[x] Executed pFUnit / CIME regression tests