feat(client): add resolveComparePair helper for history compare

apps/client/src/features/page-history/utils/resolve-compare-pair.ts

@@ -0,0 +1,18 @@
+import { ComparePair } from "@/features/page-history/atoms/history-atoms";
+
+/**
+ * Resolves which of the two selected versions is newer using their position
+ * in the history list (list is newest-first: lower index = newer).
+ */
+export function resolveComparePair(
+  historyItems: { id: string }[],
+  selection: string[],
+): ComparePair | null {
+  if (selection.length !== 2) return null;
+  const indexA = historyItems.findIndex((item) => item.id === selection[0]);
+  const indexB = historyItems.findIndex((item) => item.id === selection[1]);
+  if (indexA === -1 || indexB === -1 || indexA === indexB) return null;
+  return indexA < indexB
+    ? { newerId: selection[0], olderId: selection[1] }
+    : { newerId: selection[1], olderId: selection[0] };
+}