Simulation.subsample() raises TypeError: build_from_dataset() needs a dataset #2254

What happens

Calling subsample() on a PolicyEngine-UK simulation raises a TypeError:

Simulation.build_from_dataset() missing 1 required positional argument: 'dataset'

Reproduction

Run by the US + core hub on policyengine-uk 2.122.2, with policyengine-core at master 78a6c503 and at PolicyEngine/policyengine-core#573's head (same failure on both):

Microsimulation(dataset=...).subsample(n=10_000, seed=0, time_period=2026)

It has not been run on 2.125.1. The code below is unchanged on main, so the same failure is expected there.

Cause, read in the code

• policyengine-core, policyengine_core/simulations/simulation.py (master 78a6c503): Simulation.subsample ends with self.build_from_dataset(), called with no arguments (line 3354). Core's own build_from_dataset(self) takes none (line 1080).
• policyengine-uk, policyengine_uk/simulation.py (main e89b8abc): the override is build_from_dataset(self, dataset: Dataset) (line 455), which requires a positional dataset.
• policyengine-uk defines no subsample of its own in simulation.py or microsimulation.py, so the inherited core method runs and hits the mismatch.

This predates core#573: it fails the same way on core master.

Options

1. Override subsample in policyengine-uk so that it works with the UK dataset classes, and add a test.
2. Override it to raise a clear NotImplementedError that points to the supported way to subsample UK data.
3. Make the override's dataset argument optional and fall back to self.dataset. Check first that core's subsample (which rebuilds self.dataset through Dataset.from_dataframe) produces something the UK builder can load; the signature may not be the only mismatch.

Whichever is chosen, a test that calls subsample() on a small UK simulation would catch a regression.

Not affected

Full-sample UK totals are identical under core master and core#573.