feat: add screensaver extension pack

[keys-i]

Summary

Adds the typed Scripts UI contract for the native screensaver command from keys-i/tools#7. The wizard exposes snapshot/open, all six owned scenes, conditional Git/GIF inputs, deterministic seeds, weather controls, frame/FPS bounds, and plain/JSON output. --night is a checkbox option, so the Textual boolean-select crash path is not reintroduced.

Part of #23

Scope

• bin/
• dev/
• lib/
• sys/
• Documentation or repository settings

Testing

Commands run:

bash -n run.sh
bash run.sh --self-test
uv run dev/gui.py --self-test
git diff --check

All passed; GUI self-test covered 16 scripts, layout snapshots, keyboard behavior, and fallbacks. Mandatory fallback review returned VERDICT: PASS.

Tested systems and interpreter versions:

macOS 26.5.2 arm64
Python 3.14 via uv
Textual 8.2.8

Checklist

• The change is focused and contains no unrelated files.
• The code is clean and comments explain intent or risk.
• New or changed behaviour has a repeatable test or documented manual check.
• Help text and dependencies are current.
• Destructive behaviour and elevated permissions are clearly disclosed.
• No credentials, private data, or machine-specific paths are included.

Reviewer Notes

Stacked on science and depends on keys-i/tools#7. The pack contains no implementation or download logic; it needs the unpublished native screensaver binary on PATH. Rollback is a revert of f9506a1.