The withholding tax exemption for the employer intervention in home-work transportation costs (500 EUR/year - 41.70 EUR/month for income year 2026) was estimated from contract fields (car_atn, private_car_employee_kilometer) instead of the amounts actually taxed on the payslip. As a consequence:

- The fuel card for daily commute was fully taxed: the employee never received the exemption they are legally entitled to.
- The private car reimbursement was paid net, never added to the taxable base, yet it still granted the exemption on the yearly taxable: a double benefit.
- A bike allowance paid above the exempt rate per km (0.37 EUR/km in 2026) was never taxed.

The exemption is now computed from a new TRANSPORTATION_BASE salary rule category accumulating the taxable transportation amounts of the month:

- ATN.CAR and FUEL_CARD_COMMUTE (already taxed) are tagged with the category.
- New hidden rules CAR.PRIV.TAX and CYCLE.TAX fictively add the private car reimbursement and the above-rate bike excess to the withholding base, after ONSS and before the gross totals, while the reimbursements themselves remain paid net.
- TRANSPORT_TAX_DED becomes -min(yearly taxable, yearly threshold, 12 x monthly transportation base), including the transport lines of already validated payslips of the same month so that non-periodic (regularisation) payslips preserve the monthly totals. The termination fees variant uses the category without x12, its base being annual.

The obsolete helper _get_be_withholding_taxes_transport_deduction is removed, and the affected payslip validation expectations are updated to the corrected amounts.

task-6388103