Put FRS self-employment losses in trading_loss

[MaxGhenis]

Summary

FRS 2024-25 records self-employment losses as negative SEINCAM2 ("Any losses are recorded as such", FRS methodology glossary). The build floored them at zero, so about 74,000 people (weighted) and £1.28bn a year of losses (2024-25 prices, before uprating) disappeared from the data.

This PR splits SEINCAM2 into two non-negative inputs:

• self_employment_income: the profit, unchanged (stays non-negative, as test_non_negative_incomes requires);
• trading_loss: the loss as a positive amount.

Their difference is the reported profit. policyengine-uk needs them apart because its programmes treat a loss differently: Income Tax and tax credits set it against other income, Class 4 against trading profits only, means-tested benefits never offset it against other earnings, and HBAI counts it as negative income (PolicyEngine/policyengine-uk#2085).

Checked before relying on the sign: none of the negative values is a survey missing-value code. The five small whole-number values between −£9 and −£1 a week are all different, and each of those adults reports a loss on a self-employed job (PROFIT2).

SPI-donor rows carry no trading loss. Their self-employment profits are imputed from the SPI, which this build does not draw losses from. Keeping the FRS donor's loss would pair one respondent's loss with another taxpayer's imputed incomes. Imputing a loss instead would add random draws and shift the rest of the build. So impute_income sets their trading_loss to zero, and the FRS rows carry the losses.

Nothing else changes: the split adds no random draws, and policyengine-uk releases before #2085 read trading_loss only as a deduction from self-employment income, which is zero for every loss-maker in the data, so calibration targets are unchanged.

Tests

• test_trading_loss.py: Hypothesis properties of split_self_employment_profit for any weekly profits, including missing values: both outputs non-negative, never both positive, profit less loss equals the annualised reported profit, and monotone in the reported profit. Data checks on the built FRS (no person has both a profit and a loss) and enhanced FRS (SPI-donor rows have no loss).
• test_non_negative_incomes.py covers trading_loss.
• Adds hypothesis to the dev extras (the same line as Say which joint occupiers each FRS non-dependant family resides with #522, Supply is_claimant_or_partner from the FRS adult table #524 and Set UC gainful self-employment from the FRS main-job status #525; whichever lands second relocks).

Impact

Pending: the branch build is queued behind other builds (one heavy job at a time). The runs: policyengine-uk main and #2085 on this build with trading_loss zeroed (must match), then #2085 with the losses.

Merge

Part of the batched uk-data release (d833): approved on gates, then merged with the batch on Max's go. Lands after PolicyEngine/policyengine-uk#2085 is released, so the data's losses are read by the new rules.

Review

Pending: independent review on the final head.

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