feat(techdemo): add opaque island bridge

Add a canvas leaf-widget island to the techdemo that receives server snapshots through Effect::event/CustomEvent while keeping slhx core and runtime semantics unchanged.

req: interop/001

req: examples/001

req: examples/005

examples/techdemo/templates/control_center.heml

@@ -50,6 +50,11 @@
       <h2>Effect inspector</h2>
       <div data-slhx-slot="inspector">{+= self.inspector =+}</div>
     </article>
+    <article class="glass-card island-card" data-slhx-island="orbit" +data-island-snapshot="self.island_snapshot">
+      <h2>Opaque island bridge</h2>
+      <canvas width="360" height="180" aria-label="Animated island orbit"></canvas>
+      <p data-island-readout="">Waiting for Rust snapshot…</p>
+    </article>
     <article class="glass-card">
       <h2>Activity stream</h2>
       <div data-slhx-slot="activity">{+= self.activity =+}</div>