3.24.1: Microsimulation has no attribute _fast_cache when set_input is called

[MaxGhenis]

Summary

3.24.1 resolves the ParameterScale.clear_parent_cache issue (thanks!) but immediately exposes a second regression in the same release: Microsimulation.set_input calls self._fast_cache.pop(...) but _fast_cache isn't initialised on the instance.

Stack trace

File "policyengine_uk/simulation.py", line 420, in build_from_single_year_dataset
    self.build_from_multi_year_dataset(dataset)
File "policyengine_uk/simulation.py", line 453, in build_from_multi_year_dataset
    self.set_input(variable, year, table[variable])
File "policyengine_core/simulations/simulation.py", line 1244, in set_input
    self._fast_cache.pop((variable_name, period), None)
AttributeError: 'Microsimulation' object has no attribute '_fast_cache'

Reproduction

Any path that goes Microsimulation(dataset=...).set_input(...) hits it. policyengine-uk-data's full data build reaches this as soon as it loads a UKSingleYearDataset into a Microsimulation.

• Anchor UC/PC/CB takeup flags to FRS-reported receipt policyengine-uk-data#359 CI: https://github.com/PolicyEngine/policyengine-uk-data/actions/runs/24578803107/job/71872482893

Likely cause

Either:

• Microsimulation.__init__ (or a parent init) needs to set self._fast_cache = {} unconditionally.
• Or set_input needs to getattr(self, "_fast_cache", None) before .pop.

Blocks 3.24.x from being usable in downstream country packages.

Bonus: float-precision test regression in policyengine-uk

Separately, against 3.24.1 the UK's minimum_wage.yaml (scale parameter, absolute_error_margin: 0) now fails with 1e-7 precision wiggle:

minimum_wage@2021: [8.90999985] differs from 8.91 with an absolute margin [1.52587891e-07] > 0
minimum_wage@2025: [12.21000004] differs from 12.21 with an absolute margin [3.81469718e-08] > 0

Values look float32-rounded where they used to be float64-exact. Likely another artefact of the scale-parameter changes in 3.24.x. Same CI run: https://github.com/PolicyEngine/policyengine-uk/actions/runs/24578644004/job/71872370877

If you'd prefer these as separate issues I'll split them; filing together since they both surfaced in the same minute of PR CI output.