Tie UK year files to the source dataset and package versions they were cut from #600

Follow-up to #591 / #593, for the UK.

Problem

uk.create_datasets() names each UK year file {data_folder}/{stem}_year_{year}.h5, where the stem is dataset_logical_name(...), which drops the @revision. uk.ensure_datasets() reuses a year file whenever it exists and records its data year (DATA_YEAR_KEY), and uk.load_datasets() refuses one only when it lacks the data year. Nothing records which source a year file was cut from, or under which package versions:

• A re-pinned managed dataset. When the bundle points enhanced_frs_2024_25, populace_uk_2023 or another logical name at a new revision or sha256, year files cut from the old release keep their names and are reused.
• An edited local source. A local dataset rewritten in place (allow_unmanaged=True) keeps its year files, which still hold the old data. Two local sources with one file name in different folders share year files too.
• Another policyengine-uk or policyengine-core version. policyengine-uk projects a single-year source to later years with extend_single_year_dataset, which uprates each year by policyengine-uk parameters read through policyengine-core's parameter tree. Year files cut under one version are reused under another.

#593 fixes all three for US year files.

Proposal

Stamp each UK year file after its tables, as #593 does for the US, with the source identity (dataset_materialization.dataset_source_identity) and the installed policyengine-core and policyengine-uk versions. ensure_datasets() cuts the files again and load_datasets() refuses them unless the stamp equals the one a file cut now from the requested dataset would carry, computed without downloading or reading the source (requested_dataset_source_identity). The check reads only the stamp (and the data-year key) from each year file. Mirror #593's guards: re-check the source before stamping, refuse two sources that share a stem, and treat link cycles and malformed stamps as out of date. Share the stamp code between the countries where it stays simple.