Income tax loss_relief deducts each trading loss twice, and again in every later year the input carries into #2087

Problem

loss_relief (policyengine_uk/variables/gov/hmrc/income_tax/reliefs/loss_relief.py) is:

current_loss = person("trading_loss", period)
previous_loss = person("trading_loss", period.last_year)
return current_loss + previous_loss

It is deducted only from self-employment income (taxable_self_employment_income_deductions), and taxable_self_employment_income floors the result at zero.

1. Each loss is deducted twice. A year's loss is deducted in full in that year and again in the next, however much of it the first year used.
2. A one-off loss recurs in every later year. trading_loss is an input, and the engine carries an input into later years that are not set (auto_carry_over_input_variables in tax_benefit_system.py). Take a loss entered only for 2025. It is read as 2026's trading_loss, then 2027's, and so on, so loss_relief deducts it twice in each of those years. To avoid this, users have to enter zeros for the later years; Charge Class 4 NICs on Chapter 2 profits, after capital allowances, the trading allowance and Schedule 2 losses #1919 documents that on trading_loss.
3. The statutory limits are missing.
   • ITA 2007 s.64(2) deducts a loss in calculating net income, against general income. The deduction is for the loss-making year, the previous year, or both. The model's second deduction falls in the following year instead.
   • s.83(2) carries forward only "the part of the loss for which relief has not been given".
   • s.83(3) deducts it "only from profits of the trade". s.84 deducts it from the next year's trade profits, then each later year's, "until all the unrelieved loss is deducted".
   • The model neither tracks the part already used nor carries an excess beyond one year: whatever self-employment income does not absorb is lost.

#1919 fixes this for Class 4 NICs only:

• ni_class_4_trading_loss counts a loss only in the year it is entered for;
• ni_class_4_losses_brought_forward carries unrelieved losses forward year by year, with no ten-year spiral limit.

SSCBA 1992 Sch. 2 para. 3(4) ties Class 4 loss relief to the income tax deduction, so once income tax is fixed the two should agree.

Fix

• Count trading_loss only in the years it is supplied for, using the supplied_input helper from Charge Class 4 NICs on Chapter 2 profits, after capital allowances, the trading allowance and Schedule 2 losses #1919 (policyengine_uk/utils/supplied_inputs.py).
• Carry forward unrelieved losses against later trade profits (ss.83-84), reusing the Class 4 fold.
• Decide how to model s.64 claims against general income: current year only, or with the previous-year option.
• Property-test that total relief never exceeds the losses entered.

On impact: the #1919 PR body reports trading_loss totalling £0 in enhanced_frs_2024_25.h5@1.56.16. Check the current dataset before estimating anything.

Follow-up from #1919.