Add CI to prevent breaking changes in policyengine-us-data

[baogorek]

Context

Recent changes to microimpute have inadvertently broken policyengine-us-data's dataset generation (see #133). This highlights the need for automated testing of downstream dependencies.

Problem

Currently, developers have no way to know if their changes to microimpute will break important downstream packages like policyengine-us-data. This can lead to:

• Production failures in dependent packages
• Time spent debugging issues that could have been caught earlier
• Potential data quality issues if breaks go unnoticed

Solution

Add a GitHub Actions workflow that automatically tests policyengine-us-data when changes are made to microimpute. This ensures:

1. Developers get immediate feedback if their changes break downstream packages
2. Breaking changes can be addressed before merging
3. Better stability for the entire PolicyEngine ecosystem

Implementation

A PR will be submitted shortly that adds a downstream-compatibility.yml workflow that:

• Runs on all PRs and pushes to main
• Installs the current branch of microimpute
• Runs policyengine-us-data's PUF tests that depend on microimpute
• Warns developers if tests fail

This is especially important given that policyengine-us-data is a critical dependency for PolicyEngine's tax modeling infrastructure.

Related to: #133 (specific bug that motivated this change)