Monthlyize SSI payments and resource stocks

[MaxGhenis]

Summary

Refs #7968.

This PR:

• monthlyizes federal SSI payment calculation, SSI federal living arrangement variables, Medicaid/residential facility flags used by SSI, and state SSI supplement/SSP/OSS/OSSP payment variables
• adds ssi_federal_fiscal_year_outlays to capture SSI payment-date timing across fiscal years
• marks computed countable resource balances and asset/resource limits as STOCK, including SSI resources and TANF/resource variables that should not annualize balances
• adds regression coverage for resource stock metadata and period-invariant resource calculations

Notes

The resource-stock bug did not appear to break the standard annual SSI asset-limit eligibility path: upstream still failed SSI eligibility for a $2,100 resource case. It did affect direct monthly resource outputs and could affect monthly or annual conversions for resource intermediates, so this PR fixes the broader stock/flow issue.

State supplement tests were updated to assert monthly program outputs directly now that SSI and SSP-style payments are monthly.

Validation

• uv run pytest policyengine_us/tests/core/test_resource_stock_variables.py policyengine_us/tests/test_build_metadata.py -q (6 passed, 1 skipped)
• uv run python -m policyengine_core.scripts.policyengine_command test policyengine_us/tests/policy/baseline/gov/ssa/ssi -c policyengine_us (161 passed)
• uv run python -m policyengine_core.scripts.policyengine_command test ...state SSP/OSS/OSSP paths... -c policyengine_us (397 passed)
• uv run ruff format .
• uv run ruff check .
• git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 75.92593% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.57%. Comparing base (92fdab8) to head (c3dc940).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
...v/states/ma/dta/ssp/ma_maximum_state_supplement.py	14.28%	6 Missing ⚠️
...a/state_supplement/co_state_supplement_eligible.py	20.00%	4 Missing ⚠️
...riables/gov/states/ak/dpa/ssp/ak_ssp_claim_type.py	25.00%	3 Missing ⚠️
.../states/dc/dhcf/ossp/dc_ossp_living_arrangement.py	0.00%	3 Missing ⚠️
...ables/gov/states/ma/dta/ssp/ma_state_supplement.py	33.33%	2 Missing ⚠️
...ssi_supplement/nm_ssi_state_supplement_eligible.py	33.33%	2 Missing ⚠️
...ate_supplement/sc_ssi_state_supplement_eligible.py	33.33%	2 Missing ⚠️
.../gov/states/wa/dshs/ssp/wa_ssp_payment_category.py	0.00%	2 Missing ⚠️
...s/gov/states/ak/dpa/ssp/ak_ssp_payment_standard.py	50.00%	1 Missing ⚠️
...variables/gov/states/al/dhr/ssp/al_ssp_eligible.py	0.00%	1 Missing ⚠️
... and 13 more

Additional details and impacted files

@@             Coverage Diff              @@
##              main    #8375       +/-   ##
============================================
- Coverage   100.00%   83.57%   -16.43%     
============================================
  Files            2       89       +87     
  Lines           57     1321     +1264     
  Branches         1        4        +3     
============================================
+ Hits            57     1104     +1047     
- Misses           0      216      +216     
- Partials         0        1        +1     

Flag	Coverage Δ
unittests	83.57% <75.92%> (-16.43%)	⬇️

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