Expo-router

How switching the implementation from JS-stack to native-stack turns one import path into a hard error and silently breaks an empty-string trick — and which class of boundary the "resolver guard" and the "UIKit semantics" each belong to.

Why a declarative auth gate and an imperative router.replace inevitably flicker when they coexist — and how one hidden timing fact (derived state flips before the promise resolves) amplifies the double-driver into a visible regression.