End the Pension Credit, Child Benefit and CTC qualifying young person status for Income Support in own right

[MaxGhenis]

Fixes #2292

Stacked on #2299, #2301, #2300, #2387 and #2279. Do not merge before them. This PR edits the qualifying young person tests those PRs change. Until they merge, the diff here also shows their commits. This PR's own change:

git diff $(git merge-tree --write-tree origin/ctc-qyp-school-leaver-routes origin/qyp-terminal-date-19) origin/qyp-income-support-exclusion

Summary

A young person who receives Income Support in their own right is not a qualifying young person (QYP) for Pension Credit, Child Benefit or Child Tax Credit. The model's own-right input, receives_benefits_in_own_right, covers Universal Credit, Employment and Support Allowance and Jobseeker's Allowance, which is Universal Credit's list. Income support, which the other three programmes also name, did not end the status.

Against its base (items 1, 2 and 6 merged, on main 4d80b29 with #2279), YAML cases run as plain simulations on both (scratch/probe_E_base_vs_branch.txt):

Case	Variable	Base	This PR
2: pensioner 70, 17-year-old at school reporting £3,000 of Income Support (2025-26)	young person's Pension Credit QYP status	true	false
	young person's Child Benefit QYP status	true	false
	young person's Universal Credit QYP status	true	true
	child_minimum_guarantee_addition	£4,061.20	£0
	child_benefit_entitlement	£1,354.60	£0
3: parent 40, the same 17-year-old (2024-25)	CTC_child_element	£3,455	£0

This is a follow-up from #2279 (its "Reg. 4A(5)'s income support exclusion is also missing").

The law

• Pension Credit. SPC Regs 2002 reg. 4A(5): "A person who is receiving universal credit, a contributory employment and support allowance, a contribution-based jobseeker's allowance, an income-related employment and support allowance, an income-based jobseeker's allowance or income support is not a qualifying young person."
• Child Benefit, and through SSCBA 1992 s.142 the Housing Benefit and legacy young person. Child Benefit (General) Regulations 2006 reg. 8(2): "The condition is that the person is not in receipt, in a week, of— (a) income support, [...]".
• Child Tax Credit. CTC Regs 2002 reg. 5(4)(c): the period must not be "a period in respect of which that person receives income support, [...]".
• Universal Credit does not list it. UC Regs 2013 reg. 5(5): "A person who is receiving universal credit, an employment and support allowance or a jobseeker's allowance is not a qualifying young person."

Change

• New receives_income_support_in_own_right (Person, year): the person's own reported Income Support (income_support_reported above zero) while Income Support is in payment (gov.dwp.income_support.active, true to 31 March 2026). A dataset or situation can set it directly. It is false once a reform removes Income Support (neutralizes income_support), as is_on_income_support already treats another member's own claim; the review of this PR found the first version kept excluding the young person under such a reform.
• is_qualifying_young_person_for_pension_credit, ..._for_child_benefit and ..._for_child_tax_credit now also require that it is false. It applies to every route, as each regulation's exclusion does.
• is_qualifying_young_person_for_universal_credit is unchanged.

Why reported receipt and not the calculated award. The benefit unit's income_support is the claimant's award, and it depends on who the family's young persons are (lone-parent status and the child allowances read the Child Benefit QYP). Reading it here would be a cycle. is_on_income_support has that same dependency for claimants; for another member it reads their own report while Income Support is in payment, which is what the new variable does.

Still not separated, and noted in the docstrings: Child Benefit reg. 8 also lists incapacity benefit and tax credits; CTC reg. 5(4)(c) lists only the income-related and income-based allowances, where the existing input also catches the contributory ones.

Invariants

1. Over every combination of age, education, training, the other own-right benefits and the 16th-birthday fact: with Income Support in their own right, nobody is a Pension Credit, Child Benefit or Child Tax Credit QYP.
2. Without it, each of those three statuses is exactly what it is when the input is left alone.
3. Universal Credit status never depends on it.
4. Without it, Pension Credit and Universal Credit agree on every combination; with it, Pension Credit is the narrower.
5. By default the input is the person's own reported Income Support while Income Support is in payment (to 2025-26) and no reform removes it, and false otherwise.

Tests

• Properties, new test_qualifying_young_person_income_support_properties.py: invariants 1 to 4 over 512 combinations (8 ages, every education value, training, own-right benefits, the 16th-birthday fact), each run three ways, so 1,536 rows (input untouched, false, true), with a check that there is a status to lose; invariant 5 on Hypothesis-drawn ages, reported amounts and years from 2021 to 2028. A new test applies a reform that neutralizes income_support: the same 17-year-old student reporting £3,000 is excluded from all three statuses with Income Support and qualifies for all three once it is removed.
• YAML, new qualifying_young_person_income_support.yaml, seven cases hand-computed:

Case	Facts	Expected
1	2025-26; pensioner 70, 17-year-old at school, no Income Support	every programme's QYP; Pension Credit first-child addition £78.10 × 52 = £4,061.20; Child Benefit £26.05 × 52 = £1,354.60
2	as 1, the 17-year-old reports £3,000 of Income Support	not a Pension Credit or Child Benefit QYP, nor a legacy young person; addition and Child Benefit £0; still a Universal Credit QYP
3	2024-25; parent 40, same 17-year-old	not a CTC QYP; child element £0
4	as 3 without Income Support	child element £3,455
5	2025-26; a 16-year-old school leaver reporting Income Support	excluded from Pension Credit and Child Benefit; still a Universal Credit QYP
6	2026-27; a report of Income Support after it stopped paying	does not count
7	the input set directly	excluded

Mutation checks, each applied alone in a copy of the branch (YAML cases and property tests that fail):

Mutation	YAML cases	Property tests
Pension Credit ignores Income Support	2, 5, 7	the grid
Child Benefit ignores Income Support	2, 5, 7	the grid
Child Tax Credit ignores Income Support	3	the grid
Universal Credit excludes for Income Support too	2, 5, 7	the grid
A report counts after Income Support stopped paying	6	the default test

Run locally at c6c46e6:

• The whole stack at c6c46e6 (this PR with Count 16-year-olds outside education as UC and Pension Credit qualifying young persons until the 1 September after their 16th birthday #2279, Count only the claimant's and partner's income in the legacy means tests #2387 and the Child Benefit, CTC, terminal-date and Income Support follow-ups): policyengine-core test on the 198 YAML files that mention these programmes, 2,000 passed; pytest on the 76 Python test files that mention them is still running and will be added here.
• pytest on this PR's property file: 3 passed.
• ruff format --check and ruff check on policyengine_uk: clean.

Round 2 mutation at c6c46e6 (mutation/prE_r2.txt): receipt that ignores a reform removing Income Support fails test_no_receipt_once_a_reform_removes_income_support.

The full suites run in CI.

Microsimulation impact

Real runs on Enhanced FRS 2024-25 (uk-data 1.58.0, sha256 5ba399ea5ab9a6c7), one run each for 2024-25 to 2030-31, exact comparison of every record. Base: items 1, 2 and 6 merged (local merge 565b29c04, on main e01f2f5 with #2279 and #2387). Branch: c6c46e6. Outputs: impact/prE_r2/.

No record changes in any year. In 2024-25 and 2025-26, 60 records (about 36,000 weighted people) receive Income Support in their own right, and none is aged 16 to 19; no one aged 16 to 19 reports any Income Support (impact/prE_r1/context.txt, on the earlier base). From 2026-27 Income Support is not in payment. The PR changes household calculations (the table at the top).

Reproducer: policyengine_uk/tests/test_qualifying_young_person_income_support_properties.py::test_income_support_ends_three_statuses_and_not_universal_credit

axiom: SPC Regs 2002 reg 4A(5) TheAxiomFoundation/rulespec-uk#360 queued (its Case F and addendum cases A3 and A4 cover income support); Child Benefit (General) Regulations 2006 reg 8 TheAxiomFoundation/rulespec-uk#356 queued (addendum); CTC Regs 2002 reg 5(4)(c) TheAxiomFoundation/rulespec-uk#361 queued (addendum, Case J). Each states its blocker: the rows are in an unreleased ingest, axiom-corpus#765, and a billed encoder run needs approval.

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