Add disability basic income reform option (#866)

[vahid-ahmadi]

Summary

Implements the reform option proposed in #866: a flat per-week basic income paid to recipients of DLA, PIP, or the Universal Credit limited capability for work-related activity (LCWRA) element.

• New parameter gov.contrib.disability_basic_income.amount (£/week, default 0 — reform is inactive by default).
• New Person-level variable disability_basic_income that pays the parameter × 52 to anyone with dla > 0, pip > 0, or uc_limited_capability_for_WRA == True.
• Multiple eligibilities do not stack — the payment is per qualifying person, not per qualifying programme.
• Wired into household_benefits, gov_spending, and pre_budget_change_household_benefits so the reform shows up in fiscal aggregates and reform displays alongside the existing basic_income.

Closes #866.

Test plan

• policyengine-core test policyengine_uk/tests/policy/baseline/contrib/disability_basic_income/disability_basic_income.yaml -c policyengine_uk — 7 cases pass (default zero; DLA recipient; PIP daily living; PIP mobility; UC LCWRA; non-recipient; no-stacking).
• Existing benefit/contrib tests still pass (bi_maximum.yaml, LHA.yaml, attends_private_school.yaml, household_benefits_individual_non_dep_deduction.yaml).
• Reviewer: confirm no means-test interactions are intended for this PR. If DBI should reduce UC/HB/IS/TC like ordinary income, that's a follow-up adding interactions/include_in_means_tests per the existing UBI pattern.

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