Add example: Bun HTTP server showcasing jrx JSX-to-JSON rendering

Bun React app with HMR that demonstrates jrx's render() pipeline.
Shows JSX source, live UI via @json-render/react, and JSON output
side by side.

- example/specs/simple.tsx: flat Stack > Text + Button
- example/specs/full.tsx: nested layout with state, events,
  visibility conditions, and watchers
- Uses defineCatalog + defineRegistry from @json-render/react
- Fix package.json exports to match actual tsup output (.js/.cjs
  instead of .mjs/.js)

Co-authored-by: Ona <no-reply@ona.com>

example/specs/simple.tsx

@@ -0,0 +1,10 @@
+/** @jsxImportSource jrx */
+import { render } from "jrx";
+import { Stack, Text, Button } from "../components";
+
+export const simpleSpec = render(
+  <Stack>
+    <Text content="Hello from jrx!" />
+    <Button label="Click me" />
+  </Stack>
+);