Make Public Sandbox Functional

Why

/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.

What Changes

• Replace the placeholder /sandbox page with a functional mock-backed product surface.
• Add seeded sandbox data for a small but realistic PDPP scenario: client request, owner consent, scoped grant, records/search, revocation, and refusal evidence.
• Make the sandbox resettable in-browser and clearly simulated without sounding like project planning.
• Add inspectable API-style request/response examples so visitors can understand what an app or agent would call.
• Link the sandbox back to /reference/coverage, /reference, and /docs as evidence, not as future-work copy.

Capabilities

New Capabilities

None.

Modified Capabilities

• reference-surface-topology: strengthens the sandbox requirement from "mock-backed placeholder is acceptable" to "public sandbox SHALL provide at least one end-to-end simulated protocol walkthrough with inspectable state and reset semantics."

Impact

• apps/web/src/app/sandbox/**
• Potential shared sandbox fixtures/components under apps/web/src/components/** or apps/web/src/lib/**
• /reference/coverage evidence links and notes if sandbox demonstration status changes
• No live reference API, no real credentials, no persistent server-side user data, and no protocol wire-format change