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/partials/board_lanes.heml

@@ -0,0 +1,5 @@
+<div class="lanes">
+  <template h-for="lane in &self.lanes">
+    {+ lane +}
+  </template>
+</div>

examples/techdemo/templates/partials/hero_metrics.heml

@@ -0,0 +1,6 @@
+<div class="metrics">
+  <div class="metric"><strong>{+ self.resource_count +}</strong><span>generated resources on this page</span></div>
+  <div class="metric"><strong>{+ self.active_count +}</strong><span>active typed work items</span></div>
+  <div class="metric"><strong>{+ self.shipped_count +}</strong><span>shipped without app JS</span></div>
+  <div class="metric"><strong>{+ self.impact_score +}</strong><span>aggregate impact score</span></div>
+</div>