Bump policyengine-uk to pick up the FYE 2025 absolute poverty line #535

policyengine.py pins policyengine-uk==2.90.2, whose absolute poverty line (in_poverty_bhc / in_poverty_ahc, the default UK headline measure absolute_bhc) is HBAI's FYE 2011 line uprated by CPI.

DWP moved HBAI's absolute low-income reference year to FYE 2025, for FYE 2022 onward, in the HBAI FYE 2025 release (26 March 2026). PolicyEngine/policyengine-uk#1889 rebases policyengine-uk's line to match.

To serve it here:

1. Release policyengine-uk with #1889.
2. Publish a compatible_model_packages claim for that version on the certified UK data release (policyengine-uk-data 1.56.16), with evidence, as in Bump to policyengine-uk 2.90.2 and revert the UK certified default to enhanced FRS 2024-25 #491.
3. Certify the pair with scripts/bundle.py certify-data.
4. Run the full suite and refresh snapshots.

On 1.56.16 the new model leaves income tax, NI, UC, Child Benefit, Pension Credit, State Pension and net income within 0.1% of 2.90.2 in 2026-27 (Housing Benefit +0.8%). Absolute poverty rises by the line change; relative poverty rises from policyengine-uk#1865 (median over individuals).

The "Which measure to headline" docs (#534) describe the FYE 2011 line and need the same update.