Extended interactive playground with guided send flow

[truthixify]

Tier: L (5-7 days) | Type: feature

Context. Wave 7 landed a scan-only playground (scripts/playground/) embedded in the quickstart. Readers ask for a next step: derive a stealth key, construct a mock announcement, then scan it themselves. Turning the playground into a multi-step guided flow (derive → send → scan → withdraw) with fixture assets teaches the mental model without any wallet or network dependency.

Scope.

• Extend scripts/playground/ with three new steps and a stepper UI.
• Fixture Horizon-shaped events served from a static JSON file.
• Each step embeds inline in the relevant docs page (stellar-quickstart.mdx first step, stealth-keys.mdx derive step, fetch-announcements-stream.mdx scan step).
• Entirely client-side, no network calls, CSP-safe (iframe sandbox allow-scripts only).
• Reset button per step, share-a-permalink for a given state.

Acceptance.

• All four steps run inline on their host pages
• Zero external requests during any step (verified in DevTools network tab)
• Renders in mint dev
• Passes Compile docs snippets CI
• Playwright smoke test covers the full flow in scripts/playground/tests/

Files. scripts/playground/ (extend), guides/stellar/stellar-quickstart.mdx, api-reference/stealth-keys.mdx, api-reference/fetch-announcements-stream.mdx.