Legacy enhanced disability premium: the formula and the stored dataset flag use different qualifying benefits #1971

enhanced_disability_premium reads is_enhanced_disabled_for_benefits. Its formula is DLA care at the highest rate only. policyengine-uk-data stores a value that overrides it: AA at the higher rate, or DLA care above the highest-rate threshold (a strict > comparison), or PIP daily living at the enhanced rate. Household calculations and dataset runs therefore apply different conditions.

The enhanced disability premium conditions should be re-verified on legislation.gov.uk before choosing a definition:

• HB Regs 2006 Sch 3 para 15;
• IS Regs 1987 Sch 2 para 13A;
• ESA Regs 2008 Sch 4 para 7;
• JSA Regs 1996 Sch 1 para 15A.

The things to establish are the qualifying benefits and rates (for example DLA care highest, PIP daily living enhanced, AFIP, the ESA support component), the age limit and the couple rate.

Proposed fix: an explicitly named qualifying variable, like receives_severe_disability_premium_qualifying_benefit in #1946; align the uk-data flag or stop storing it; add YAML tests and properties; run the Enhanced FRS impact with real runs.

Found while reviewing #1946.

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