Expo prebuild and config plugins

Continuous Native Generation: ios/ and android/ are build output of app.config.ts, its config plugins and the installed native packages. Every native change is a config key or one line in the plugin list; local plugins are typed, idempotent and tested; the generated folders stay out of git and out of every script's reach; a committed snapshot of the introspected native config makes each native change visible in review. Two checkers prove it. Leaving this model is a recorded decision.

Rules that must hold