Salary sacrifice NI measures charge the whole amount returned at one rate #2361

salary_sacrifice_pension_ni_employee multiplies the whole amount returned to employment income (salary_sacrifice_returned_to_income) by one rate: the main rate if employment_income is at or below the upper earnings limit, otherwise the additional rate. salary_sacrifice_pension_ni_employer multiplies it by the employer rate.

Neither equals the change in the Class 1 liability the model computes (ni_class_1_employee, ni_class_1_employer) when the amount returned enters earnings:

• An amount that straddles the upper earnings limit is charged entirely at the additional rate. With 2025 thresholds and a £2,000 cap, pay of £48,000 with £6,000 sacrificed returns £4,000 and takes earnings to £52,000. The liability rises by £216.20 (8% up to £50,269.96, 2% above it); the variable reports £80.
• An amount below, or straddling, the primary threshold (employee) or the secondary threshold (employer) is charged in full. Pay of £1,500 with £5,000 sacrificed gives earnings of £4,500, below the £5,000 secondary threshold: the employer liability is £0 and the variable reports £450.
• The age conditions are ignored. The employee variable charges people under 16 or over State Pension age (ni_liable), and the employer variable charges under-16s (ni_class_1_secondary_liable).
• The employee variable compares employment_income with the limit, not Class 1 earnings (ni_class_1_income, which also includes statutory sick, maternity and paternity pay).

Both are reporting measures. No tax total reads them: the amount returned is already in employment_income, so ni_class_1_employee and ni_class_1_employer charge it.

Found by the independent specification of the tax-head partition written for #2235 (its defect 7).

Fix: make each variable the complete Class 1 liability on earnings less the liability on the same earnings without the amount returned, with YAML cases and a property test of that identity.