docs: add generated terminal capture gallery

[2chevskii]

Summary

• Add a committed VHS GIF demo to the README.
• Publish 90 built-in theme screenshots and five glyph-set screenshots in a lazy-loaded VitePress gallery.
• Generate tapes and gallery documentation from the local module registry, with a deterministic PNG stale-content check in GitHub Actions.

Validation

• Pester tests (49 passed)
• Documentation build
• Module manifest validation
• Manual visual review of representative light, dark, and Emoji captures
• Final GitHub Actions capture verification (in progress: https://github.com/2chevskii/gly/actions/runs/29133603049)

Checklist

• The change is focused and does not include unrelated modifications.
• Tests and documentation have been updated where needed.
• I have read and agree to follow the Code of Conduct.