Prepare policyengine-us-data for .py-owned stack releases

[anth-volk]

Goal

Remove policyengine-us-data from the old bundle orchestration path and make it a normal package/artifact that can be cited and pinned by the .py stack manifest.

Work

• Remove PR/release checks that install or validate against policyengine-bundles bundle contracts.
• Keep the data package release process focused on publishing policyengine-us-data and its release manifest/artifacts.
• Ensure .py can reference the exact package version, Python markers, release manifest URL, and default dataset URI needed for citation and verification.
• After a data release, open a stack-only PR in policyengine.py when that data version should become part of the official stack.
• Update docs or workflow messages that tell maintainers to refresh old release bundles.

Acceptance criteria

• A policyengine-us-data release does not trigger or require old bundle publication.
• policyengine.py can include or exclude policyengine-us-data from stack extras without changing this repo.
• Lightweight stack verification can check the package version and relevant release/data URIs.