Native (Fabric) portal/re-parenting primitive for React Native. Covers the 4-export API (PortalProvider, PortalHost, Portal, usePortal), the auto-appended "root" host, hostName exact-match/silent-local-fallback semantics, teleport (re-parenting without unmount) for shared-element-style transitions, modal window isolation, and placeholder sizing since teleported content reserves no space at its origin.
