test+docs: full-app browser integration test and the isomorphic counter example

The integration test runs the counter through the real browser path in
happy-dom: start(), store events, raf-batched morph, DOM click handlers,
emit coalescing. examples/counter is the Phase 2 exit criterion: one app
module mounted zero-build in a browser via import map and string-rendered
by node examples/counter/render.js.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014NgfSjHE11oFpoSnLVKLXd

examples/counter/render.js

@@ -0,0 +1,5 @@
+// Server-side render of the exact same app module the browser mounts:
+//   node examples/counter/render.js
+import createApp from './app.js'
+
+process.stdout.write(createApp().toString('/') + '\n')