Test the public-page copy buttons offline

[thangldw]

Outcome

Add a DOM-level regression test for docs/proofline-page.js so both copy-button paths remain reliable without network access.

Acceptance criteria

• The success path writes exactly proofline demo stale-decision to the clipboard and shows the copied status.
• The failure path shows the demo command visibly when clipboard access is unavailable.
• The test covers both copy buttons.
• No browser dependency, remote asset, or external service is added.
• Relevant test and quality commands pass.

Scope

Keep this limited to the public project page script and its tests. Do not change the application clipboard behavior.