feat(react): remount islands on hot module replacement

Disposes and re-creates island roots after a source change. Island state
resets by design; Fast Refresh needs a Babel/SWC transform plus a
runtime and is out of scope for v1.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/react/src/index.ts

@@ -6,3 +6,4 @@ export { IslandErrorBoundary } from "./error-boundary.tsx";
 export type { IslandErrorBoundaryProps } from "./error-boundary.tsx";
 export { islandRootCount, mountIslands, unmountIslands } from "./island-runtime.ts";
 export type { MountOptions } from "./island-runtime.ts";
+export { remountIslands } from "./island-runtime.ts";