feat(FN-1544): add viewport-gated loading and lightweight memo comparison

- Add viewport-gated fetching to TaskCard component so only visible cards load their data
- Implement lightweight memo comparison to prevent unnecessary re-renders
- Add lazy enable gates to useSessionFiles and useTaskDiffStats hooks with caching
- Add comprehensive tests for useSessionFiles and useTaskDiffStats hooks
- Document viewport-gated loading patterns in memory and dashboard-load performance docs

packages/dashboard/app/hooks/useSessionFiles.ts

@@ -8,11 +8,39 @@ interface UseSessionFilesResult {
   loading: boolean;
 }
 
-export function useSessionFiles(taskId: string, worktree: string | undefined, column: string, projectId?: string): UseSessionFilesResult {
+interface UseSessionFilesOptions {
+  /** Enable fetching when true (default). Suppresses fetches for offscreen cards. */
+  enabled?: boolean;
+}
+
+/**
+ * Fetches session files for tasks with active worktrees.
+ *
+ * @param taskId - Task identifier
+ * @param worktree - Worktree path (undefined = no worktree)
+ * @param column - Current task column
+ * @param projectId - Optional project identifier
+ * @param options.enabled - When false, no fetch is made and returns empty/stable state
+ */
+export function useSessionFiles(
+  taskId: string,
+  worktree: string | undefined,
+  column: string,
+  projectId?: string,
+  options: UseSessionFilesOptions = {},
+): UseSessionFilesResult {
+  const enabled = options.enabled ?? true;
   const [files, setFiles] = useState<string[]>([]);
   const [loading, setLoading] = useState(false);
 
   useEffect(() => {
+    // Disabled state: return stable empty state without fetching
+    if (!enabled) {
+      setFiles([]);
+      setLoading(false);
+      return;
+    }
+
     if (!taskId || !worktree || !ACTIVE_COLUMNS.has(column)) {
       setFiles([]);
       setLoading(false);
@@ -40,7 +68,7 @@ export function useSessionFiles(taskId: string, worktree: string | undefined, co
     }
 
     void load();
-  }, [taskId, worktree, column, projectId]);
+  }, [taskId, worktree, column, projectId, enabled]);
 
   return { files, loading };
 }