feat(layout): Plan 2.5 P1 foundation — APP_NAVIGATION + walkNavTree + AppBreadcrumb

Per RFC-WS-PRIMEVUE-PLAN-2-5 §8 step 1. Foundation scaffolding only —
no shell fixes, no Public Sans removal, no useShellUiStore changes
(P2–P6 scope).

Implements:
- theme darkModeSelector verified at '.dark' (already correct in
  plugins/primevue/index.ts — config site is here, not theme.ts).
- src/config/navigation.ts: APP_NAVIGATION registry per AD-2.5-B1
  (Dashboard entry only — v2-dashboard is the only v2 route today).
- src/composables/useBreadcrumb.ts: walkNavTree pure helper +
  useNavBreadcrumb composable per AD-2.5-B1. The legacy meta-based
  useBreadcrumb is preserved (consumed by AppTopbar, P1 may not
  touch AppTopbar); P4 retires it and renames useNavBreadcrumb.
- src/components-v2/layout/AppBreadcrumb.vue: layout primitive
  wrapping PrimeVue Breadcrumb, consuming useNavBreadcrumb.
- Tests: walkNavTree (4 specs, co-located), AppBreadcrumb mount
  (2 specs, tests/component/layouts/).

Suite 564 → 570 (+6, all new specs green). vue-tsc clean. Scoped
ESLint clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

apps/app/src/components-v2/layout/AppBreadcrumb.vue

@@ -0,0 +1,53 @@
+<script setup lang="ts">
+/**
+ * AppBreadcrumb — layout primitive per RFC-WS-PRIMEVUE-PLAN-2-5 AD-2.5-B1.
+ *
+ * Consumes `useNavBreadcrumb` (registry-driven; data derives from
+ * APP_NAVIGATION + the current route name). Wired into AppTopbar in P5
+ * (RFC §5.2 Fix 2) — standalone in P1.
+ *
+ * Naming note: imports `useNavBreadcrumb` (not the literal `useBreadcrumb`
+ * the AD specifies) because the legacy meta-based `useBreadcrumb` is still
+ * consumed by AppTopbar; P1 may not touch AppTopbar (P4–P6 scope). P4
+ * retires the legacy API and renames `useNavBreadcrumb` → `useBreadcrumb`.
+ */
+
+import { computed } from 'vue'
+import Breadcrumb from 'primevue/breadcrumb'
+import { useNavBreadcrumb } from '@/composables/useBreadcrumb'
+
+const crumbs = useNavBreadcrumb()
+
+const items = computed(() =>
+  crumbs.value.map(c => ({
+    label: c.label,
+    route: c.routeName ? { name: c.routeName } : undefined,
+  })),
+)
+</script>
+
+<template>
+  <Breadcrumb
+    :model="items"
+    :pt="{
+      root: { class: 'border-none p-0 bg-transparent' },
+      list: { class: 'gap-1' },
+    }"
+  >
+    <template #item="{ item }">
+      <RouterLink
+        v-if="item.route"
+        :to="item.route"
+        class="text-sm text-surface-600 hover:text-surface-900 dark:text-surface-400 dark:hover:text-surface-100 transition-colors"
+      >
+        {{ item.label }}
+      </RouterLink>
+      <span
+        v-else
+        class="text-sm font-medium text-surface-900 dark:text-surface-100"
+      >
+        {{ item.label }}
+      </span>
+    </template>
+  </Breadcrumb>
+</template>