feat(board): canonical board definition and BoardView (T2)

Encode the board as blocks of stacked-section columns (C1) and render it as
the ASCII-styled DOM grid (C2) with hover highlight and a select emit. Layout
mirrors the physical board (Client Satisfaction above Product Architecture;
Macroplan above Defect Visualisation, both left of Feature Kanban) and boxes
use true ISO 216 proportions (1:√2, each step up scaled by √2). Both views
render from the one definition; tests pin section presence, click, hover,
layout order, and a snapshot.

app/pages/index.vue

@@ -1,6 +1,13 @@
+<script setup lang="ts">
+const selected = ref<string | null>(null)
+</script>
+
 <template>
   <main>
     <h1>Andon — report a board problem</h1>
-    <p>The interactive board lands in Task 2.</p>
+    <BoardView @select="(id) => (selected = id)" />
+    <p v-if="selected">
+      Selected section: <code>{{ selected }}</code> — the defect form arrives in Task 5.
+    </p>
   </main>
 </template>