Surface UK Populace Zenodo DOI in bundle provenance

[anth-volk]

PR #409 attempted to surface the UK Populace preservation DOI (10.5281/zenodo.20678518) in the old release-manifest TRO path via pe:preservationDoi.

That PR has been superseded by the merged .py-owned bundle work (#425/#427/#438), which certifies UK Populace through src/policyengine/data/bundle/manifest.json instead of the old src/policyengine/data/release_manifests/uk.json pathway.

We should port the preservation DOI support into the current bundle system rather than merging PR #409 as-is.

Acceptance criteria:

• The current UK bundle data release metadata records the Zenodo preservation DOI for the certified Populace UK release, if that DOI is still the correct preserved artifact for the currently certified build.
• Bundle TRO generation emits the preservation DOI, e.g. as pe:preservationDoi, when the data release manifest/bundle metadata includes it.
• Tests cover DOI propagation from bundle/data-release metadata into the generated TRO.
• The old release-manifest pathway is not restored.