Call the person who follows an External interaction's link the link holder, not the visitor

Closes https://www.drupal.org/project/orchestra/issues/3631391

The module called the person who follows an External interaction's link "the visitor". Everybody on a website is a visitor, and the word suggests someone outside the organization, which they need not be. What sets them apart is that the link is their only credential.

• Where a person must be named: link holder (French: détenteur du lien).
• Elsewhere, the wording speaks of the link itself: "the page the link opens", "an outcome chosen through the link", "the link chain", "a segment of a run that the link follows".
• An anonymous site user who is not specifically following a link is "an anonymous user".
• "Visitor link" is now "External interaction link", and the "visitor-interaction settings" are the workflow's "External interaction settings".

Covers the interface strings and their French translations, the docs (docs/*.md, module READMEs), code comments, and test names and helpers (assigned_after_link, openExternalInteractionLink(), the testTheExternalInteractions…, …TheLinkHolder… and …ItsHolder… names). Comment paragraphs and Markdown prose the rewording made too long were reflowed as whole blocks.

Wording only: no stored configuration, data or API uses the word.

AI-Generated: Yes (Claude Code was used to write the change and this description. I reviewed them.)