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/Entities/LendRecord.cs

@@ -0,0 +1,72 @@
+namespace LibraryApp.Domain.Entities;
+
+public class LendRecord
+{
+    public LendRecordId Id { get; private set; }
+    public BookInstanceId BookInstanceId { get; private set; }
+    public MemberId MemberId { get; private set; }
+    public LendCode Code { get; private set; }
+    public DateTime LendDate { get; private set; }
+    public DateTime? ReturnDate { get; private set; }
+
+    public LendRecord(BookInstanceId bookInstanceId, MemberId memberId, LendCode code)
+        : this(new LendRecordId(string.Empty), bookInstanceId, memberId, code, DateTime.UtcNow, null)
+    { }
+
+    public LendRecord(LendRecordId id, BookInstanceId bookInstanceId, MemberId memberId, LendCode code, DateTime lendDate, DateTime? returnDate)
+    {
+        Id = id;
+        BookInstanceId = bookInstanceId;
+        MemberId = memberId;
+        Code = code;
+        LendDate = lendDate;
+        ReturnDate = returnDate;
+    }
+
+    public LendRecord MarkAsReturned()
+    {
+        if(ReturnDate != null)
+            throw new InvalidOperationException("This lend record is already marked as returned.");
+        ReturnDate = DateTime.UtcNow;
+        return this;
+    }
+
+    public LendRecord UpdateLendDate(DateTime newLendDate)
+    {
+        if(ReturnDate != null)
+            throw new InvalidOperationException("Cannot update lend date for a record that is already marked as returned.");
+        LendDate = newLendDate;
+        return this;
+    }
+
+    public LendRecord UnmarkAsReturned()
+    {
+        if(ReturnDate == null)
+            throw new InvalidOperationException("This lend record is not marked as returned.");
+        ReturnDate = null;
+        return this;
+    }
+}
+
+public record LendRecordId : DbId
+{
+    public LendRecordId(string id) : base(id) {}
+}
+
+/// <summary>
+/// A short, human-friendly code written on the physical lend slip alongside the member's name.
+/// Must be unique per lend record and no longer than 8 characters.
+/// </summary>
+public record LendCode
+{
+    public string Code { get; init; }
+
+    public LendCode(string code)
+    {
+        if (string.IsNullOrWhiteSpace(code))
+            throw new ArgumentException("Lend code cannot be empty.");
+        if (code.Length > 8)
+            throw new ArgumentException("Lend code cannot exceed 8 characters.");
+        Code = code.ToUpperInvariant();
+    }
+}