Define Microplex mp-300k default switch and rollback behavior

Parent plan: https://github.com/PolicyEngine/microplex-us/discussions/16

Goal: define how policyengine-us will switch the default national dataset from eCPS to Microplex `mp-300k` once release gates pass.

Deliverables:
- Define feature flag or configuration behavior for selecting `mp-300k` versus eCPS.
- Define fallback behavior if the Microplex artifact is unavailable or incompatible.
- Document the default-switch release checklist and rollback path.
- Coordinate version constraints with policyengine-us-data once the beta loader exists.

Exit criterion: policyengine-us can make `mp-300k` the default dataset in a controlled release with an explicit rollback path.