Migrate API v1 to PolicyEngine bundle 4.18.7

[anth-volk]

API v1 should move to the certified PolicyEngine .py 4.18.7 bundle and consistently use the packaged bundle manifest for certified package and dataset metadata.

Required work:

• Update the API v1 policyengine[models] pin to 4.18.7.
• Derive bundled policyengine-core, policyengine-us, and policyengine-uk versions from the .py manifest.
• Expose certified Populace default datasets in metadata.
• Reject deprecated short dataset names while preserving full custom dataset URI passthrough.
• Keep UK auxiliary breakdown artifacts documented as postprocessor inputs until they are separately certified.
• Add unit-scoped coverage for dataset validation and bundle-default canonicalization.