refactor(core): consolidate shared app foundations

lib/focus/hub_vertical_navigation.dart

@@ -0,0 +1,33 @@
+/// Routes vertical D-pad movement between ordered hub rows.
+///
+/// A valid target and the bottom boundary are consumed. The top boundary can
+/// either invoke an explicit focus handoff or propagate to the row's
+/// `onNavigateUp` callback.
+bool navigateVerticalHubRows({
+  required int hubCount,
+  required int hubIndex,
+  required bool isUp,
+  required void Function(int targetIndex) requestFocus,
+  void Function()? onTopBoundary,
+  void Function()? onBottomBoundary,
+  bool propagateTopBoundary = false,
+}) {
+  if (hubCount <= 0) return false;
+
+  final targetIndex = isUp ? hubIndex - 1 : hubIndex + 1;
+  if (targetIndex < 0) {
+    if (onTopBoundary != null) {
+      onTopBoundary();
+      return true;
+    }
+    return !propagateTopBoundary;
+  }
+
+  if (targetIndex >= hubCount) {
+    onBottomBoundary?.call();
+    return true;
+  }
+
+  requestFocus(targetIndex);
+  return true;
+}