refactor(techdemo): scope card handles to partials

Move board card and lane actions onto their reusable partial templates, register the scoped generated handles directly, and remove hidden shell-level handle anchors and manual handle fields from the rendered view data.

req: component/003

req: view/001

req: examples/001

examples/techdemo/templates/partials/issue_lane.heml

@@ -1,4 +1,4 @@
-<section +class="self.class" +data-lane="self.lane_id" +data-hid="self.move_handle" data-slhx-on="drop">
+<section +class="self.class" +data-lane="self.lane_id" data-slhx-handle="move_to_lane" data-slhx-on="drop">
   <h3>{+ self.title +}</h3>
   <p>{+ self.description +}</p>
   <template h-for="card in &self.cards">