Expand Universal Credit: childcare element + capital/tariff income

[vahid-ahmadi]

Addresses #40

Builds on the already-structured calculate_universal_credit (standard allowance, child/disabled-child, LCWRA, carer, housing, work allowance + 55% taper, unearned income) by porting two elements that were missing relative to the Python gov/dwp/universal_credit/ tree.

Scope (this PR)

• Childcare element (uc_childcare_element_monthly) — 85% of registered childcare costs for working benunits, capped at the DWP 2025/26 monthly maxima (£1,031.88 one child / £1,768.94 two-or-more). UC Regs 2013 regs.31–35.
• Capital test — £6,000 lower disregard, £16,000 upper cliff (nil award), and £4.35/month tariff income per complete £250 of capital above £6,000, treated as unearned income. UC Regs 2013 reg.18, reg.72 (uc_tariff_income_annual + cliff in calculate_universal_credit).
• New UniversalCreditParams fields are #[serde(default)] so all prior-year YAMLs still deserialise; correct 2025/26 figures set in parameters/2025_26.yaml.
• LEGISLATIVE_REFERENCE.md sections 3.6 (childcare) and 3.7 (capital) added; changelog fragment added.

Already structured on main (in scope of #40, present before this PR)

Standard allowance (4 bands), child element with two-child limit, disabled-child element, LCWRA element, carer element, housing element (LHA-capped), work allowance (lower/higher) + 55% earnings taper, unearned income, take-up gating via would-claim flags.

Remaining

• Transitional protection / managed migration top-up calculation (migration routing exists; the protected-amount top-up does not).
• Full LCWRA via the Work Capability Assessment (proxied from PIP/DLA/ESA flags).
• Bedroom-tax (under-occupancy) reduction for social renters.

Tests

cargo build and cargo test pass (195 tests). New worked-example tests: couple over-25 + 2 children (no earnings), couple + 2 children with £12k earnings taper, £16k capital cliff, £6.5k tariff income (£104.40/yr), and the childcare element (coverage, cap, and work condition).

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