Implement .py-owned pip-native PolicyEngine stacks

[anth-volk]

Goal

Move stack/bundle ownership into policyengine.py, with the .py release version also serving as the stack version. Users should be able to install a validated PolicyEngine stack with normal pip commands such as pip install "policyengine[full]==X.Y.Z".

Work

• Add a stack manifest owned by this repo that records the exact PolicyEngine-owned package versions included in a stack.
• Generate pyproject.toml optional dependencies from that manifest for extras such as models, data, full, us-full, and uk-full.
• Add runtime/CLI support for policyengine stack show, policyengine stack verify, and policyengine stack cite.
• Add a stack-only PR workflow: update the manifest, regenerate derived files, run lightweight verification, and publish via the normal .py release path.
• Add release assets for each .py release: stack manifest JSON, constraints file, citation text, and verification output.
• Keep verification lightweight: exact installed versions, importability/module discovery, and reachable release-manifest/data URIs where applicable.
• Decide how artifact-only packages are represented, especially UK data if it remains unavailable as a pip-installable package.

Acceptance criteria

• pip install "policyengine[models]==<version>" installs the exact model/core versions recorded by the stack.
• policyengine stack verify --extra models can run in CI quickly.
• A stack-only PR can be opened without touching downstream repos.
• The normal .py release publishes stack metadata as release assets.
• Documentation explains install commands, citation, and how to open a stack update PR.