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";

packages/react/src/island-runtime.ts

@@ -100,3 +100,16 @@ export function unmountIslands(root: ParentNode): void {
     roots.delete(element);
   }
 }
+
+/**
+ * Dev-only: dispose and re-create 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. `unmountIslands` clears each element from the
+ * root registry, so the following `mountIslands` is not short-circuited by the
+ * already-mounted guard.
+ */
+export async function remountIslands(root: ParentNode, options: MountOptions): Promise<void> {
+  unmountIslands(root);
+  await mountIslands(root, options);
+}