Pin HuggingFace dataset version to avoid stale cache

[MaxGhenis]

Problem

policyengine-uk downloads the enhanced FRS dataset from HuggingFace without pinning a version:

hf://policyengine/policyengine-uk-data/enhanced_frs_2023_24.h5

hf_hub_download uses an etag check to detect updates, but if the check times out (default 10s) or fails silently, it falls back to the stale cached version in ~/.cache/huggingface/hub/. This has caused real bugs — a stale cache missing highest_education led to KeyError crashes in economic_assumptions.py.

Fix

Pin the dataset version in the URL using the existing @version syntax:

hf://policyengine/policyengine-uk-data/enhanced_frs_2023_24.h5@1.40.1

Both HF repos (policyengine-uk-data and policyengine-uk-data-private) already publish version tags (e.g. 1.40.1). The download code in simulation.py already parses @version and passes it as the revision parameter to hf_hub_download.

Benefits

• Reproducible: each policyengine-uk release is tied to a specific data version
• No stale cache: a new version = new cache key, so there's no ambiguity
• Explicit upgrades: when policyengine-uk-data publishes a new version, policyengine-uk bumps the pin deliberately

Implementation

1. In simulation.py, append @{version} to the HF URL (currently around line 149)
2. When policyengine-uk-data releases a new version, update the pinned version in policyengine-uk