Publish and load Microplex mp-30k-uk beta artifact

[MaxGhenis]

Umbrella: PolicyEngine/microplex-uk#2

Once microplex-uk produces a passing mp-30k-uk beta candidate, add the policyengine-uk-data publication/loading path.

Scope:

• Define artifact naming and metadata conventions for mp-30k-uk.
• Preserve eFRS as a pinned incumbent comparator.
• Expose enough release metadata for rollback and reproducibility.
• Do not switch defaults until Microplex replacement gates pass.

Acceptance:

• policyengine-uk-data can publish or load an mp-30k-uk beta artifact through the normal UK data path.