Updated the domain model with entities, repos and domain services. Looks good. I think I addressed all the user stories however if I fell short we'll revise as needed.

LibraryApp.Domain/BookCountReport.cs

@@ -0,0 +1,30 @@
+using LibraryApp.Domain.Entities;
+
+namespace LibraryApp.Domain;
+
+public class BookCountReport
+{
+    public ShelfId ShelfId { get; }
+    public DateTime CountedAt { get; }
+    public IReadOnlyList<BookInstance> MissingInstances { get; }
+    public int TotalExpected { get; }
+    public int TotalFound { get; }
+
+    public BookCountReport(
+        ShelfId shelfId,
+        DateTime countedAt,
+        IReadOnlyList<BookInstance> allInstancesOnShelf,
+        IReadOnlyList<BookInstanceBarcode> scannedBarcodes)
+    {
+        ShelfId = shelfId;
+        CountedAt = countedAt;
+        TotalExpected = allInstancesOnShelf.Count;
+
+        var scannedSet = scannedBarcodes.Select(b => b.Barcode).ToHashSet();
+        MissingInstances = allInstancesOnShelf
+            .Where(i => !scannedSet.Contains(i.Barcode.Barcode))
+            .ToList();
+
+        TotalFound = TotalExpected - MissingInstances.Count;
+    }
+}