refactor(techdemo): compose page with hemplate views

Move the techdemo page shell and remaining board/inspector/hero boundaries to hemplate-owned views instead of Rust raw HTML or placeholder replacement. Keep rendered HTML assertions DOM-aware with scraper.

req: html_safety/002

req: view/001

req: test/005

examples/techdemo/templates/control_center.heml

@@ -5,7 +5,7 @@
       <h1>A Linear-class work system without a frontend framework.</h1>
       <p class="lede">Create, inspect, advance, and stream work through native HTML, generated typed resources, and compact EffectBatches. The UI feels app-grade; the model stays server-owned and boring.</p>
     </div>
-    <div class="hero-panel" data-slhx-slot="hero_metrics">__HERO__</div>
+    <div class="hero-panel" data-slhx-slot="hero_metrics">{+= self.hero =+}</div>
   </section>
 
   <nav class="topology" data-slhx-slot="nav">
@@ -41,18 +41,18 @@
         <span>Generated slots + keyed cards</span>
         <strong>generated resources, no selectors</strong>
       </div>
-      <div data-slhx-slot="board">__BOARD__</div>
+      <div data-slhx-slot="board">{+= self.board =+}</div>
     </section>
   </section>
 
   <section class="insight-grid">
     <article class="glass-card">
       <h2>Effect inspector</h2>
-      <div data-slhx-slot="inspector">__INSPECTOR__</div>
+      <div data-slhx-slot="inspector">{+= self.inspector =+}</div>
     </article>
     <article class="glass-card">
       <h2>Activity stream</h2>
-      <div data-slhx-slot="activity">__ACTIVITY__</div>
+      <div data-slhx-slot="activity">{+= self.activity =+}</div>
     </article>
     <article class="glass-card glow">
       <h2>Server push</h2>