perf(router-core): reduce fallback lifecycle bundle overhead

We replace declare _lifecycleEnd?: number with _lifecycleEnd = 0 to fix the SyntaxError thrown by Babel in all three e2e test environments. The declare keyword was introduced as a bundle-size optimization, but Babel's @babel/plugin-transform-typescript cannot process declare class fields when class property transforms run in a conflicting order. Both 0 and undefined are falsy, so the runtime semantics — zero meaning the full branch participates in lifecycle callbacks — are preserved identically.