Date the mixed-age couple Pension Credit exclusion and model the SI 2019/37 saving

changelog.d/pension-credit-mixed-age-saving.fixed.md

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+- Date the Pension Credit exclusion of mixed-age couples (State Pension Credit Act 2002 s.4(1A)) from 15 May 2019, and model the SI 2019/37 article 4 saving. Before the change, mixed-age couples could claim Pension Credit and pension-age Housing Benefit, and now take that route rather than Universal Credit. Afterwards, a couple entitled to Pension Credit or pension-age Housing Benefit on 14 May 2019 keeps them while it stays entitled. The saving is a new input, `has_mixed_age_couple_pension_credit_saving`, which by default is inferred from reported Pension Credit or pension-age Housing Benefit without reported Universal Credit, where the older member was born by 1954. A family on the Pension Credit route is no longer eligible for Universal Credit.

docs/book/programs/gov/dwp/housing-benefit.ipynb

@@ -21,8 +21,8 @@
     "\n",
     "`housing_benefit_eligible` requires the benunit to be either in **social housing** or eligible for the **Local Housing Allowance** rate (broadly, private renters), with assessable capital at or below the working-age (or pension-age) capital limit, and to reach Housing Benefit by one of two routes:\n",
     "\n",
-    "- **Pension age**: every adult in the benunit has reached the qualifying age for State Pension Credit (State Pension age). New Housing Benefit claims are open to them ([SI 2014/1230 reg. 6A(4)](https://www.legislation.gov.uk/uksi/2014/1230/regulation/6A)), and Universal Credit is not, so neither a previous claim nor `would_claim_uc` matters.\n",
-    "- **Continuing award**: any other benunit was **already claiming** Housing Benefit (proxied by a positive `housing_benefit_reported`) and is **not claiming Universal Credit**. Working-age and mixed-age families cannot make new claims; they claim the Universal Credit housing element instead. For mixed-age couples the reported award stands in for the saving for couples who were entitled to Housing Benefit or Pension Credit on 14 May 2019 ([SI 2019/37 art. 4](https://www.legislation.gov.uk/uksi/2019/37/article/4)).\n",
+    "- **Pension age**: the claimant and any partner have reached the qualifying age for State Pension Credit (State Pension age), or the family is a mixed-age couple that keeps the pension-age route: one entitled to Pension Credit or pension-age Housing Benefit on 14 May 2019 and ever since ([SI 2019/37 art. 4](https://www.legislation.gov.uk/uksi/2019/37/article/4)), or any mixed-age couple not reported on Universal Credit before 15 May 2019 (`meets_pension_credit_age_conditions`). New Housing Benefit claims are open to them ([SI 2014/1230 reg. 6A(4) and (5)](https://www.legislation.gov.uk/uksi/2014/1230/regulation/6A)), and Universal Credit is not, so neither a previous claim nor `would_claim_uc` matters.\n",
+    "- **Continuing award**: any other benunit was **already claiming** Housing Benefit (proxied by a positive `housing_benefit_reported`) and is **not claiming Universal Credit**. Working-age families, and mixed-age couples without the saving, cannot make new claims; they claim the Universal Credit housing element instead.\n",
     "\n",
     "The pension-age test counts the same benunit members as Universal Credit's working-age test, and the continuing-award route requires not claiming Universal Credit, so no benunit receives both. Working-age awards outside specified or temporary accommodation ended on 1 July 2026 in Great Britain ([SI 2025/1148 art. 7](https://www.legislation.gov.uk/uksi/2025/1148/article/7)); PolicyEngine does not yet model that. Claims for specified or temporary accommodation, which remain open at any age, are not modelled.\n",
     "\n",

policyengine_uk/parameters/gov/dwp/pension_credit/mixed_age_couples/excluded.yaml

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+description: Whether a member of a mixed-age couple, where one partner has not reached the qualifying age for State Pension Credit, is excluded from State Pension Credit. The exclusion also bars the couple from new pension-age Housing Benefit claims.
+values:
+  2003-10-06: false
+  2019-05-15: true
+metadata:
+  unit: bool
+  label: Mixed-age couple exclusion from Pension Credit in force
+  reference:
+    - title: State Pension Credit Act 2002 s.4(1A)
+      href: https://www.legislation.gov.uk/ukpga/2002/16/section/4
+    - title: SI 2019/37 art. 3 (appointed day 15 May 2019)
+      href: https://www.legislation.gov.uk/uksi/2019/37/article/3
+    - title: SR 2019/4 art. 3 (Northern Ireland, appointed day 15 May 2019)
+      href: https://www.legislation.gov.uk/nisr/2019/4/article/3

policyengine_uk/parameters/gov/dwp/pension_credit/mixed_age_couples/saving_latest_birth_year.yaml

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+description: Latest birth year of the older member of a mixed-age couple for the couple to keep Pension Credit under the SI 2019/37 article 4 saving. The older member must have reached the qualifying age for State Pension Credit by 14 May 2019, which means being born on or before 5 February 1954. With whole-year ages the model admits the whole of 1954.
+values:
+  2019-05-15: 1954
+metadata:
+  unit: year
+  label: Latest birth year for the mixed-age couple Pension Credit saving
+  reference:
+    - title: SI 2019/37 art. 4(1)
+      href: https://www.legislation.gov.uk/uksi/2019/37/article/4
+    - title: Pensions Act 1995 Sch 4 para 1 (pensionable age by date of birth)
+      href: https://www.legislation.gov.uk/ukpga/1995/26/schedule/4
+    - title: State Pension Credit Act 2002 s.1(6) (qualifying age)
+      href: https://www.legislation.gov.uk/ukpga/2002/16/section/1