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/tsconfig.json

@@ -0,0 +1,9 @@
+{
+  "extends": "../../tsconfig.json",
+  "compilerOptions": {
+    "noEmit": true,
+    "jsx": "react-jsx",
+    "jsxImportSource": "react"
+  },
+  "include": ["src/**/*.ts", "src/**/*.tsx", "test/**/*.ts", "test/**/*.tsx"]
+}