Extend MD EITC minimum-age disregard to married childless filers

[DTrim99]

Fixes #8934.

Problem

Maryland lets any filer who fails only the federal EITC minimum-age test claim the state EITC on a pro forma federal credit computed as if the § 32 minimum-age requirement did not apply (2025 Resident Booklet, Instruction 18: "There is no minimum age requirement for taxpayers without a qualifying child").

PolicyEngine already models the pro forma credit — federal_eitc_without_age_minimum — but wired it only into the unmarried-childless path. The married-or-has-child branch used the actual age-gated federal credit (eitc), so a married childless couple under 25 received $0.

Fix

Feed federal_eitc_without_age_minimum into both married-branch formulas (md_married_or_has_child_non_refundable_eitc, md_married_or_has_child_refundable_eitc) for childless filers, gated on eitc_child_count == 0:

childless = tax_unit("eitc_child_count", period) == 0
federal_eitc = where(
    childless,
    tax_unit("federal_eitc_without_age_minimum", period),
    tax_unit("eitc", period),
)

The gate matters: eitc carries takeup and filer conditions that federal_eitc_without_age_minimum omits, so an unconditional swap would have silently changed results for with-child filers. Childless-gating leaves with-child filers on the actual federal EITC (no age test applies to them) and matches the existing unmarried-childless treatment.

Reproduction (from the issue)

MD, MFJ, ages 23/24, no dependents, $10,476 wages (2025):

Variable	Before	After
eitc	0	0
federal_eitc_without_age_minimum	649	649
md_refundable_eitc	0	292.05 (45% × 649)

Tests

• Updated md_married_or_has_child_{non_refundable,refundable}_eitc.yaml: childless cases now assert the age-disregarded base, plus new with-child regression guards confirming those filers still use the actual federal eitc.
• New integration test md_married_childless_age_disregard.yaml reproducing the issue household end-to-end.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f9e58e7) to head (43bf665).
⚠️ Report is 153 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #8936   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            3         2    -1     
  Lines           55        36   -19     
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- Hits            55        36   -19     

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

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[MaxGhenis]

Reviewed: the childless gate on both married-branch formulas matches MD Instruction 18, and the integration household cross-validates against the Maine 18-24 expansion merged in #8932 — identical $10,476 MFJ household, identical $649 pro-forma federal credit, independently implemented (ME 50% childless match → $324.50; MD 45% refundable → $292.05 vs TaxAct $325 Line 22). Merging on green.