Add federal CCDF infrastructure for scaling CCAP to all states

[hua7450]

Summary

Closes #7764

Creates shared federal-level CCDF variables and parameters so state CCAP implementations don't duplicate identical federal rules. This is the foundation for scaling child care subsidies to all 50 states + DC.

Changes

1. New: is_ccdf_immigration_eligible_child variable

• Reuses existing federal gov/dhs/immigration/qualified_noncitizen_status parameter (8 USC 1641) and adds a citizen check — no duplicate parameter file
• Covers: CITIZEN, LPR, REFUGEE, ASYLEE, DEPORTATION_WITHHELD, CUBAN_HAITIAN_ENTRANT, CONDITIONAL_ENTRANT, PAROLED_ONE_YEAR
• States that follow the federal list can reuse this directly instead of creating their own
• 11 test cases covering all qualified + ineligible statuses (UNDOCUMENTED, DACA, TPS)

2. New: child_care_subsidies federal aggregator

• Follows the same pattern as tanf.py — maintains a list of state subsidy variables and sums them
• Replaces the hardcoded state list in childcare_expenses.py (was ["CA", "CO", "NE", "MA"])
• Adding a new state now means adding one line to STATE_CHILD_CARE_SUBSIDY_VARIABLES
• Note: TX CCS is not included — it has a pre-existing entity broadcast issue in microsim that needs to be resolved separately

3. Refactored: childcare_expenses.py

• Removed inline state list and string manipulation
• Now references child_care_subsidies aggregator directly

What's NOT in this PR

• No refactoring of existing state implementations (CA, CO, MA, NE) — they continue to work as-is
• No changes to RI CCAP (PR Implement Rhode Island CCAP #7739) — that will reference these federal variables in a follow-up

Test plan

• is_ccdf_immigration_eligible_child tests pass (11 cases: 8 qualified statuses + 3 ineligible)
• Existing CCDF tests still pass
• Existing state childcare subsidy tests still pass (CA, CO, MA, NE)
• Microsimulation tests pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (752fae4) to head (19ec305).
⚠️ Report is 52 commits behind head on main.

Additional details and impacted files

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+ Hits            13        33   +20     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[PavelMakarchuk]

LGTM after yaml file