Publish and load Microplex mp-300k beta dataset

[MaxGhenis]

Parent plan: PolicyEngine/microplex-us#16

Goal: add the policyengine-us-data publication and loader path needed for a Microplex mp-300k beta dataset.

Deliverables:

• Define the H5 naming and metadata contract for Microplex mp-300k artifacts.
• Add a loader/publication path for the mp-300k beta artifact.
• Preserve eCPS as the incumbent baseline and rollback option.
• Document that eCPS is the incumbent comparator, not the target truth source.

Exit criterion: policyengine-us-data can publish and load mp-300k through a normal dataset interface without changing the default yet.

[MaxGhenis]

Umbrella discussion: PolicyEngine/microplex-us#16