Prepare policyengine-core for .py-owned stack releases

[anth-volk]

Goal

Make policyengine-core a normal pinned component of the .py-owned PolicyEngine stack, without depending on policyengine-bundles contract tests or publication hooks.

Work

• Remove old bundle contract CI/tests that import or validate against policyengine-bundles.
• Keep local metadata tests focused on policyengine-core runtime/build metadata, not cross-repo bundle schemas.
• Ensure policyengine.py can pin an exact core version and verify the installed package quickly.
• Keep release workflow focused on publishing policyengine-core; stack inclusion should happen through a follow-up PR to policyengine.py.

Acceptance criteria

• policyengine-core releases do not trigger or require old bundle publication.
• policyengine.py can include an exact policyengine-core pin in stack extras.
• Stack verification can validate the installed core version without importing old bundle tooling.