Council Tax Reduction: assess every claim's scheme and exemption on its own applicant #2157

Since #2006, a joint Council Tax Reduction claim (families sharing the rent) is assessed on the applicant's own family. Every other claim still takes its scheme and non-dependant exemption from household flags (council_tax_reduction_household_has_pensioner, council_tax_reduction_household_has_non_dep_exemption), read from the benefit unit's claimant and partner.

Three outcomes on main (6b366b281) are wrong. Each case is 2026, Maidstone or Cardiff, with a £1,800 bill:

1. A report on top of a paying simulated claim. A pension-age head with no income and a working-age sharer are jointly liable, each for half; the sharer reports £1,500. Main pays £900 simulated plus the £1,500 report, £2,400 against the £1,800 bill. SI 2012/2885 Sch 1 para 7(3)-(4) limits a jointly liable applicant's maximum reduction to their share.
2. A household head who applies alone. An 80-year-old grandmother heads the household and is liable. Her benefit unit's claimant and partner are 17-year-old parents. Main does not treat her as a pensioner (SI 2012/2885 reg 3), so she gets nothing under Maidstone's unmodelled working-age scheme. Her applicant exemption also counts the young mother's PIP (Sch 1 para 8(6)).
3. A household where no family can claim. A 17-year-old head reports £500 in Cardiff. Main gives the family the Welsh scheme's simulated nil, so the report is lost; in Maidstone it is kept.

#2015 assesses every claim on its own applicant and partner and reconciles simulated and reported reductions within a household.