UK: structural reforms gated on `*.in_effect=True` silently no-op via `simulation_modifier` path

Inizia da policyengine_uk/reforms/reforms.py e src/policyengine/tax_benefit_models/uk/model.py:187-202; elenca i gate gov.contrib.*.in_effect e traccia quando viene eseguito create_structural_reforms_from_parameters in Microsimulation.init. Confronta il percorso UK con tests/test_us_microsim_structural_reforms.py::test__gov_contrib_gate_runs_cleanly. Il lavoro è completato quando un test di regressione UK dimostra che il gate funziona attraverso il percorso di costruzione previsto, modificando l’implementazione se viene confermato il bug latente.

Flagged during subagent review of #300.

The UK model applies reforms via simulation_modifier_from_parameter_values after Microsimulation construction (src/policyengine/tax_benefit_models/uk/model.py:190,198), using p.update() on parameters. This means any UK structural reform that is gated on a *.in_effect=True parameter — where the structural reform class only activates inside Microsimulation.__init__'s call to create_structural_reforms_from_parameters — would silently no-op.

US recently hit this class of bug (#300) when gov.contrib.ctc.*.in_effect=True reforms crashed because the structural reform's variables were registered on the per-sim system but populations were built from the module-level system. The UK variant is subtly different: the structural reform never gets applied at all because p.update() happens post-construction.

Evidence

• policyengine_uk/reforms/reforms.py contains at least one in_effect gate (confirmed via grep).
• UK uses simulation_modifier at src/policyengine/tax_benefit_models/uk/model.py:187-202, which runs after Microsimulation.__init__ has already decided which structural reforms to apply (based on base-parameter in_effect=False).

What to check

1. Enumerate UK gov.contrib.*.in_effect parameters. If any exist and activate structural reforms, this is a real latent bug.
2. Write a regression test along the lines of tests/test_us_microsim_structural_reforms.py::test__gov_contrib_gate_runs_cleanly for a UK gate.
3. If confirmed, the fix is to pass UK reforms at Microsimulation(reform=...) construction time (same pattern US uses) instead of via post-construction p.update().

Not blocking v4.0.1; file as a v4.0.2 / v4.1 candidate.