refactor(cohort): implement lightweight two-phase cohort creation

[johnpaulalex]

Description:

Refactors the fates_cohort_type constructor from a monolithic procedure into a clean two-stage lifecycle (InitRaw for memory/identity initialization and UpdateDerivedState for allometry/parameter queries). Adds lightweight CreateBare constructor for unit tests, enabling test fixtures to instantiate valid cohort linked lists without initializing global parameter tables or running heavy allometry routines.

Specific notes

Collaborators:

• @jpalex

Linked issues addressed, if any:

• None

Preview of Child Branches

• fix+test/biogeophys-accum-fluxes+cohort-create (fix(fates): fix c13disc_acc weighting order and add EDAccumulateFluxes unit tests #1602): Fixes c13disc_acc weighting order calculation bug in EDAccumulateFluxesMod, adds zero-denominator underflow protection, introduces CreateTestSite and SetTestFluxes test factory helpers in FatesFactoryMod, and refactors test_EDAccumulateFluxes.pf unit test suite.

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: 5efa6fbfd173afe85d523431e75c8a6ec493b242
FATES baseline hash-tag: 36c49d7e

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