Make Public Sandbox Functional

/sandbox currently reads like a placeholder for work we intend to do. Public reviewers and prospective implementers need a concrete, useful surface that lets them experience PDPP with simulated data immediately, without connecting real accounts or running the reference stack.

The current /sandbox route is intentionally honest but not useful: it says the sandbox is a future mock-backed educational surface. That satisfied the route-topology change, but it is not good enough for an internal demo or a public reviewer. A visitor should be able to click through a simulated PDPP story and understand why the protocol matters without reading the whole spec first.

Any public sandbox surface SHALL be mock-backed, resettable, and clearly labeled as simulated. It SHALL teach protocol flows and API shapes without collecting real platform credentials or presenting itself as a live owner reference instance. The primary /sandbox entry point SHALL provide at least one functional, end-to-end simulated PDPP walkthrough rather than only describing planned future sandbox behavior.