Two-child limit targets declared at 2026 are structurally unreachable (limit abolished April 2026)

The nine two-child limit targets are declared at 2026 only, but the limit is abolished from 2026-04-06, so uc_is_child_limit_affected is structurally zero that year. They can never bind.

policyengine_uk_data/targets/sources/dwp.py:203-305:

Target(
    name="dwp/uc/two_child_limit/households_affected",
    variable="uc_is_child_limit_affected",
    values={2026: 453_600},   # <- never reachable
)

policyengine-uk parameters/gov/dwp/universal_credit/elements/child/limit/child_count.yaml:

2017-04-06: 2
2026-04-06: .inf   # Budget 2025 abolition

Model output confirms it (enhanced FRS 2023-24, v1.56.16): every TCL target computes to 0.0 in 2026.

These are April 2025 statistics. Moved to 2025, the model overshoots:

Next steps

1. Move all nine targets from 2026 to 2025.
2. Add a registry test that fails when a target is declared in a year where its variable is structurally zero — this class of dead target should not be silently possible.
3. The +34/+42% overshoot is then a live calibration signal, likely tied to the 3+ children undercount in #466.

cc @MaxGhenis @vahid-ahmadi