Docs: add a detailed, illustrated external-interaction internals page

The External interaction docs page is an overview; it does not explain the URL interaction mechanism in depth. This adds a specialized, illustrated deep-dive page, docs/interaction-internals.md, covering how a public token-bearing URL drives a parked step and why it is safe.

Contents:

• The request lifecycle (a sequence diagram through the dispatcher, token, resolver, plugin, gateway, engine).
• Anatomy of the capability token, as an SVG illustration: the instance/token/expiry/signature fields and the HMAC keyed by the site private key + hash salt.
• Minting and validating (the sign() and open() paths, hash_equals, expiry).
• The dispatcher decision tree, link scope (instance vs branch) as an SVG, and the blast radius.
• A threat-by-threat security table.
• The one-plugin, two-doorways model (bearer vs identity) and the continuation-handle resolver chain.
• The Webform integration (resume-on-submit binding and chaining).
• Reference tables for routes and key classes.

Static inline SVG plus Mermaid diagrams (no JavaScript beyond the theme's Mermaid), added to the nav after External interaction. Verified with mkdocs build --strict.