refactor(kanban): render board cards with hemplate

Move Kanban board/column/card rendering from Rust raw HTML assembly into hemplate partial views. Keep handler effects returning generated slot payloads and cover rendered structure with scraper.

req: html_safety/002

req: view/001

req: test/005

examples/kanban/templates/partials/board_card.heml

@@ -0,0 +1,10 @@
+<article class="card" +data-key="self.id">
+  <strong>{+ self.title +}</strong>
+  <menu>
+    <button h-if="self.left_disabled" type="button" +data-hid="self.left_handle" +data-card-id="self.id" disabled="disabled">←</button>
+    <button h-else type="button" +data-hid="self.left_handle" +data-card-id="self.id">←</button>
+    <button h-if="self.right_disabled" type="button" +data-hid="self.right_handle" +data-card-id="self.id" disabled="disabled">→</button>
+    <button h-else type="button" +data-hid="self.right_handle" +data-card-id="self.id">→</button>
+    <button type="button" +data-hid="self.delete_handle" +data-card-id="self.id">Delete</button>
+  </menu>
+</article>

examples/kanban/templates/partials/board_column.heml

@@ -0,0 +1,6 @@
+<section class="column">
+  <h2>{+ self.title +}</h2>
+  <template h-for="card in &self.cards">
+    {+ card +}
+  </template>
+</section>

examples/kanban/templates/partials/board_columns.heml

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