Define PolicyEngine-UK switch and rollback path for mp-30k-uk

[MaxGhenis]

Umbrella: PolicyEngine/microplex-uk#2

Prepare the model-side path for using Microplex UK data once the data gates pass.

Scope:

• Identify any loader/API assumptions that are specific to eFRS.
• Define the default-switch criteria for mp-30k-uk.
• Define an explicit rollback path to eFRS.
• Keep eFRS available as an incumbent comparator during beta.

Acceptance:

• PolicyEngine-UK can switch to mp-30k-uk only after documented gates pass, with a tested rollback path.