Expo project bootstrap

Turns an empty (or docs-only) folder into an Expo app with the canonical stack: one app at the repo root with Expo Router in src/app/, every root config, the gate scripts, the dependency policy, app.config.ts with three variants for iOS and Android, a starter route and screen, and placeholder art. Everything comes from templates that were installed and run end to end; every gate is green on day one, and a checker proves the skeleton stays that way. The multi-package form is fenced under "When multi-package" in references/project-layout.md.

Rules that must hold