test_built_total_gains fails on reduced (TESTING=1) builds of main #535

policyengine_uk_data/tests/test_cgt_band_donors.py::test_built_total_gains fails on reduced (TESTING=1, 32-epoch) builds of main, so PR CI can go red on PRs that have nothing to do with capital gains.

Measured locally on main b45c373 (TESTING=1 build): £242.6bn of above-AEA gains against HMRC's £65.9bn, a relative error of 268%. The reduced-build tolerance is 0.5 × 5.0 = 250%.

Seen in PR CI:

• severe-disability-flag-tax-credit-definition, run 36784812236: 255%. The same branch passed two hours earlier.
• Keep FRS-reported dividends and key them on person_id #498: 274% and 260%.

Full builds pass. Production-setting builds of main and #498 (512 epochs) pass all eight test_cgt_band_donors.py tests at the strict bounds.

So the reduced-build check sits at its threshold. It catches calibration noise rather than the "order-of-magnitude pathologies" its comment describes. Options:

• loosen the reduced-build tolerance with evidence of its spread across seeds;
• run more epochs for this check;
• skip it under TESTING, as test_built_cgt_taxpayer_count already does.