Separate GitHub and PyPI project descriptions

[max-giraffe]

Summary

• make the GitHub README viewer screenshot follow the current branch asset
• use a dedicated PyPI long description with an unlinked banner and no redundant PyPI call-to-action
• load the updated viewer screenshot from the maintained main-branch asset on future package releases
• add the PyPI description review to the release checklist

Verification

• 36 tests passed in the project virtual environment
• source distribution and wheel built successfully
• strict twine checks passed for both distributions
• wheel metadata verified to contain both current image URLs and no PyPI self-link