Harden state nonrefundable credit guardrails

[MaxGhenis]

Summary

• document the shared *_potential vs applied-credit convention for ordered state nonrefundable credits
• add helper-level invariant tests for remaining-liability calculation and ordered capping
• add a code-health guard that flags new downstream consumers of applied credit variables unless they have been reviewed intentionally

Why

PR #7984 split several state credits into potential and applied forms. The Maryland refundable CDCC regression showed that downstream formulas can silently read the applied amount when a form or worksheet really refers to the pre-ordering amount. This PR makes that distinction explicit and adds tests around the specific bug class.

Impact

This does not change any state credit amounts directly. It hardens the shared helper semantics and adds regression coverage so future split-credit refactors are less likely to reintroduce Maryland-style downstream-consumer bugs.

Validation

• pytest policyengine_us/tests/core/test_non_refundable_credit_cap.py policyengine_us/tests/code_health/test_non_refundable_credit_downstream_consumers.py -q
• python -m policyengine_core.scripts.policyengine_command test -c policyengine_us policyengine_us/tests/policy/baseline/gov/states/md/tax/income/credits/md_refundable_cdcc.yaml policyengine_us/tests/policy/baseline/gov/states/de/tax/income/credits/eitc/de_eitc.yaml policyengine_us/tests/policy/baseline/gov/states/de/tax/income/credits/eitc/refundability_calculation/de_income_tax_if_claiming_non_refundable_eitc.yaml policyengine_us/tests/policy/baseline/gov/states/ky/tax/income/credits/family_size_tax_credit/ky_family_size_tax_credit.yaml policyengine_us/tests/policy/baseline/gov/states/la/tax/income/credits/child_care_expense_credit/la_school_readiness_tax_credit.yaml policyengine_us/tests/policy/baseline/gov/states/ny/tax/income/credits/ny_eitc.yaml policyengine_us/tests/policy/baseline/gov/states/va/tax/income/credits/eitc/va_eitc.yaml policyengine_us/tests/policy/baseline/gov/states/va/tax/income/credits/eitc/va_eitc_person.yaml policyengine_us/tests/policy/baseline/gov/states/va/tax/income/credits/eitc/refundability_calculation/va_income_tax_if_claiming_non-refundable_eitc.yaml

Closes #7991.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.00%. Comparing base (0671338) to head (29d1ea6).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #7992      +/-   ##
===========================================
- Coverage   100.00%   96.00%   -4.00%     
===========================================
  Files            4        1       -3     
  Lines           72       25      -47     
  Branches         0        3       +3     
===========================================
- Hits            72       24      -48     
- Partials         0        1       +1     

Flag	Coverage Δ
unittests	96.00% <ø> (-4.00%)	⬇️

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