Reset negative HBAI household income to zero before housing costs, as HBAI does #2401

hbai_household_net_income is never reset at zero, but HBAI resets a negative before-housing-costs income to zero and derives after-housing-costs income from the reset figure:

Negative incomes BHC are reset to zero, but negative AHC incomes calculated from the adjusted BHC incomes are possible. Where incomes have been adjusted to zero BHC, income AHC is derived from the adjusted BHC income.

(HBAI Background Information and Methodology report FYE 2025, "Negative incomes", published 26 March 2026.)

Households whose deductions (Council Tax, Income Tax, NICs, pension contributions, maintenance paid) exceed their income get a negative HBAI income in the model today. That feeds equivalised income, the income deciles (where such households are given decile -1 and left out), poverty gaps, means and inequality measures built on equiv_hbai_household_net_income. #2085 (trading losses as negative HBAI income) makes negative incomes more common.

Fix: reset the BHC figure at zero, keep the figure before the reset for diagnostics and for the labour supply responses (which need the actual budget constraint), derive AHC from the reset figure, and check every consumer.