Add UK H5 compatibility checks for Microplex exports

[MaxGhenis]

Umbrella: #2

Add an automated compatibility gate before any UK Microplex candidate can be release-track.

Scope:

• Validate PolicyEngine-UK H5 loading.
• Check household, benefit-unit, person, and tax-unit IDs and joins.
• Check weights, periods, dtypes, missing-value conventions, and entity row counts.
• Produce a machine-readable report for the dashboard.

Acceptance:

• mp-30k-uk artifacts cannot be marked release-track without a passing H5 contract report.