feat(react): add per-island error boundary

A crashed island renders its error in dev and nothing in prod, leaving
the surrounding server-rendered page intact.

Island .tsx sources carry an explicit @jsxImportSource react pragma: the
repo's root tsconfig points jsxImportSource at @wrnexus/core, so without
it island JSX compiles to WRNexus's string renderer instead of React
elements.

Tests render on the client via createRoot rather than a server renderer,
because React error boundaries do not engage during SSR — and islands
are client-only regardless.

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

packages/react/src/error-boundary.tsx

@@ -0,0 +1,43 @@
+/** @jsxImportSource react */
+import { Component, type ErrorInfo, type ReactNode } from "react";
+
+export interface IslandErrorBoundaryProps {
+  name: string;
+  development: boolean;
+  children: ReactNode;
+}
+
+interface IslandErrorBoundaryState {
+  error: Error | null;
+}
+
+/**
+ * Contains island failures locally: a crashed island must never blank the
+ * surrounding server-rendered page.
+ */
+export class IslandErrorBoundary extends Component<
+  IslandErrorBoundaryProps,
+  IslandErrorBoundaryState
+> {
+  override state: IslandErrorBoundaryState = { error: null };
+
+  static getDerivedStateFromError(error: Error): IslandErrorBoundaryState {
+    return { error };
+  }
+
+  override componentDidCatch(error: Error, info: ErrorInfo): void {
+    console.error(`[wrnexus] island '${this.props.name}' failed to render`, error, info);
+  }
+
+  override render(): ReactNode {
+    const { error } = this.state;
+    if (!error) return this.props.children;
+    if (!this.props.development) return null;
+    return (
+      <div data-wrn-island-error={this.props.name} style={{ padding: "0.75rem" }}>
+        <strong>{`Island '${this.props.name}' failed`}</strong>
+        <pre>{error.stack ?? error.message}</pre>
+      </div>
+    );
+  }
+}