feat: public path fallback

[DanilaBeseda]

Added client.publicPathFallback - it lets the runtime switch to the next configured public path when an async chunk
fails to load, and stay on it for all later chunk loads on the page.

Details:

1. Backup paths can be scoped to specific hosts (hosts: string | RegExp), matched against location.hostname.
2. Every switch or exhaustion dispatches a typed window CustomEvent, so consumers can report failures to their own
   monitoring.
3. Disabled by default, and has no effect in dev mode, SSR builds, or with moduleFederation.