Mark the benefit units that share their household's rent

[MaxGhenis]

Fixes #511

Stacked on #506 (base frs-boarder-lodger-rent), which is stacked on #503. Model side: PolicyEngine/policyengine-uk#2006.

Summary

Adds the benefit-unit input liable_for_share_of_household_rent. In FRS shared households (HHSTAT 2), it is true for each benefit unit after the first whose adults report the rent they pay (SRENTAMT > 0) or housing benefit (HBOTHAMT > 0). rent (HHRENT) is unchanged and stays the rent of the whole dwelling. policyengine-uk#2006 splits it per person liable (UC Regs 2013 Sch 4 para 24(4); HB Regs 2006 reg 12B(4)).

Why not BURENT

The FRS derived variable BURENT is meant to split HHRENT across the units of a shared household. In the raw 2024-25 and 2023-24 EUL files it does not:

• Units 2+. Between them they are given all of HHRENT, in proportion to their SRENTAMT. The ratio of BURENT to HHRENT × SRENTAMT share is p10 0.998, p90 1.004 (169 units, 2024-25).
• Unit 1. It gets a further amount: BURENT/HHRENT has a median of 1.5 and a p90 of 2.0.
• Totals. BURENT summed over a shared household is about 2.5 times HHRENT: £12.7bn against £5.0bn a year in 2024-25. This is the whole of the gap between unit-1 BURENT (£96.6bn) and HHRENT (£93.9bn). Conventional households match exactly.
• Likely cause. RENT is blank in the EUL for every shared household. That is consistent with the spec's TOTRENT having lost its RENT term.

HHRENT and SRENTAMT are coherent. HHRENT ≈ n/(n−1) × the other units' summed SRENTAMT (median 1.5; 2.0 where there are two units), which is consistent with equal shares.

In the base build (FRS 2024-25, survey weights)

	Value
Benefit units flagged	177 (613k weighted)
Households with a flagged unit	97 (306k weighted)
Rent of those households (HHRENT)	£4.97bn a year

Raw cross-check: 178 later units of shared households have BURENT > 0.

Tests

policyengine_uk_data/tests/test_frs_shared_household_rent.py (7 cases, synthetic frames, no survey data) covers:

• later paying units;
• HBOTHAMT alone;
• non-positive SRENTAMT;
• conventional households;
• a couple reporting on one partner's record;
• no leakage across households.

Impact

The build. Enhanced build of this branch at a9e3371: production settings, sha256 ed1a29912656, finished in 53 minutes, exit 0. The only errors were the existing HMRC salary-sacrifice CSV download, which returns 410.

This PR on its own. It changes no output until policyengine-uk reads the new input. policyengine-uk ignores dataset columns it has no variable for.

With policyengine-uk#2006. On this build, same weights, the head of #2006 against its base (#1896) gives:

• Universal Credit: +£30.9m in 2025-26 and +£33.6m in 2026-27.
• UC housing costs element: sharers £0 → £664m, boarders and lodgers £0 → £147m, household heads −£335m (2025-26).
• Housing Benefit and Council Tax Reduction: unchanged.
• Nothing changed outside households with a sharer, boarder or lodger.

The full tables are in the #2006 description.

axiom: n/a: data input (the rules are in policyengine-uk#2006; rulespec-uk#406)

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