Interactive scanner demo embed in docs

[truthixify]

Extra labels: feature, dx
Tier: L (5-7 days) | Type: feature

Context. Static code snippets don't build intuition. Embed an interactive scanner (like a playground) inside guides/stellar/stellar-quickstart.mdx that runs against a canned event batch.

Scope.

• iframe-embedded playground (source in docs repo) that lets a reader paste a scan key + run the SDK's scanner against fixture events.
• Sandboxed (no wallet, no real Horizon calls).
• CSP-safe.

Acceptance.

• Playground renders on the quickstart page
• Runs entirely client-side against fixture data
• No third-party JS

Files. guides/stellar/stellar-quickstart.mdx, scripts/playground/ (new).