Add Node.js test harness for SX WASM kernel with happy-dom

Boots the full WASM kernel (js_of_ocaml mode) + 24 .sxbc modules
into a happy-dom DOM environment. Provides helpers for SX eval,
DOM queries, island hydration, and click simulation.

Smoke test covers: kernel eval, DOM manipulation via SX, component
rendering, signals (reset!/swap!/computed), effects, and island
hydration with button rendering — 19/19 pass in ~2.5s.

Known limitation: reactive text nodes inside islands don't re-render
after click (signal updates but cek-reactive-text doesn't flush).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -0,0 +1,8 @@
+{
+  "devDependencies": {
+    "happy-dom": "^20.8.9"
+  },
+  "dependencies": {
+    "playwright": "^1.58.2"
+  }
+}