Add South Carolina fully refundable EITC contrib reform

[DTrim99]

What this does

Adds a Child Poverty Impact Dashboard contrib reform that converts South Carolina's nonrefundable EITC to fully refundable for all filers, activated by gov.contrib.states.sc.child_poverty_impact_dashboard.eitc.in_effect. This is the SC half of #8682.

SC's EITC is 125% of the federal EITC, capped per filer at $200 from 2026, and is nonrefundable by default (applied only up to remaining state income tax liability), so zero-liability filers receive nothing today.

Approach

Mirrors the corrected Utah/Missouri/Ohio refundability reforms (#8645 / #8642 / #8657) — same principles, no reintroduced bugs:

• Pay the potential, not the capped credit. sc_fully_refundable_eitc returns sc_eitc_potential (uncapped at liability), so the full credit is paid as a refund. sc_eitc is capped at liability and zeroes out for exactly the low-liability filers refundability helps.
• Rebuild the non-refundable bucket correctly. sc_non_refundable_credits re-runs ordered_capped_state_non_refundable_credits over the ordered list with sc_eitc filtered out — no add("<param-path string>") bug, and no sum − sc_eitc overstatement when the bucket binds.
• Clear inherited adds before adding a formula. sc_refundable_credits sets adds = None; subtracts = None (the baseline computes via adds) and returns the standard refundable credits plus sc_fully_refundable_eitc.

The statutory per-filer cap (gov.states.sc.tax.income.credits.eitc.max) is preserved and remains separately adjustable — the dashboard exposes it as its own lever.

New files

• policyengine_us/reforms/states/sc/child_poverty_eitc/sc_fully_refundable_eitc_reform.py (+ __init__.py)
• policyengine_us/parameters/gov/contrib/states/sc/child_poverty_impact_dashboard/eitc/in_effect.yaml
• policyengine_us/tests/policy/reform/sc_fully_refundable_eitc.yaml
• registration in policyengine_us/reforms/reforms.py; changelog fragment

Verification

• 4 YAML cases pass: full potential paid as refundable at zero liability; no double-count when liability is present; the $200 (2026) cap is respected; no effect outside SC.
• End-to-end: setting …child_poverty_impact_dashboard.eitc.in_effect = true on a 2026 SC filer with a federal EITC of $100 and zero SC liability yields sc_fully_refundable_eitc = $125, sc_refundable_credits = $125, and +$125 household net income (baseline pays $0).

Part of #8682.