UC rebalancing reforms don't take effect unless the Scenario applies before the data load #2252

What's wrong

Simulation.__init__ runs the Universal Credit rebalancing modifier (scenarios/uc_reform.py) after building the simulation. A Scenario whose parameter_changes apply after the data load (the default) is applied later, so the modifier has already set uc_LCWRA_element from the baseline parameters. As a result, reforms of these parameters silently do nothing:

• gov.dwp.universal_credit.rebalancing.new_claimant_health_element;
• gov.dwp.universal_credit.rebalancing.active;
• any parameter the protected amount reads.

Reproducer (main e89b8abc)

There are 40 identical single LCWRA claimants in one situation; on main, the draw gives 5 of them the new-claimant rate. The reform sets the new-claimant rate to £250 from 2026-27 ({"2026": 250}).

The documentation example in docs/book/policy/uc-rebalancing.md ("Changing the health element amount for new claimants") also keys the change "2026-01-01". A single-day key never reaches the 30 April fiscal-year sampling date, so the example does nothing in either mode. #2242 fixes the same key problem in the standard allowance example.

Expected

A reform of a rebalancing parameter should change the health element whichever way the Scenario is applied. One option is to compute the element in the uc_LCWRA_element formula, from the claimant's status (#2250) and the parameters, rather than overwriting it with set_input at build time.

Found while reviving the UK leg of PolicyBench, alongside #2247 and #2250.