⚡ Cache Python stub builds

[burgholzer]

🤖 AI text below 🤖

Description

Stub generation now uses the existing shared sccache policy: pull requests read the cache, main writes it, and the job verifies that compilation used it. Python test scheduling remains unchanged.

Validation: all workflow lint hooks, including actionlint and zizmor, pass. Hosted cache behavior still requires CI validation.

GPT-6 via Codex implemented and validated this change at the maintainer's request.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

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[burgholzer]

Should also help to speed up CI