Implement checkpoint modes with docs/tests and reorganize project file layout

tests/CBDD.Tests/Types/ObjectIdTests.cs

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+using ZB.MOM.WW.CBDD.Bson;
+using Xunit;
+
+namespace ZB.MOM.WW.CBDD.Tests;
+
+public class ObjectIdTests
+{
+    /// <summary>
+    /// Verifies new object identifiers are 12 bytes long.
+    /// </summary>
+    [Fact]
+    public void NewObjectId_ShouldCreate12ByteId()
+    {
+        var oid = ObjectId.NewObjectId();
+
+        Span<byte> bytes = stackalloc byte[12];
+        oid.WriteTo(bytes);
+
+        bytes.Length.ShouldBe(12);
+    }
+
+    /// <summary>
+    /// Verifies object identifiers round-trip from their binary form.
+    /// </summary>
+    [Fact]
+    public void ObjectId_ShouldRoundTrip()
+    {
+        var original = ObjectId.NewObjectId();
+
+        Span<byte> bytes = stackalloc byte[12];
+        original.WriteTo(bytes);
+
+        var restored = new ObjectId(bytes);
+
+        restored.ShouldBe(original);
+    }
+
+    /// <summary>
+    /// Verifies object identifier equality behavior.
+    /// </summary>
+    [Fact]
+    public void ObjectId_Equals_ShouldWork()
+    {
+        var oid1 = ObjectId.NewObjectId();
+        var oid2 = oid1;
+        var oid3 = ObjectId.NewObjectId();
+
+        oid2.ShouldBe(oid1);
+        oid3.ShouldNotBe(oid1);
+    }
+
+    /// <summary>
+    /// Verifies object identifier timestamps are recent UTC values.
+    /// </summary>
+    [Fact]
+    public void ObjectId_Timestamp_ShouldBeRecentUtc()
+    {
+        var oid = ObjectId.NewObjectId();
+        var timestamp = oid.Timestamp;
+
+        (timestamp <= DateTime.UtcNow).ShouldBeTrue();
+        (timestamp >= DateTime.UtcNow.AddSeconds(-5)).ShouldBeTrue();
+    }
+}

tests/CBDD.Tests/Types/ValueObjectIdTests.cs

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+using ZB.MOM.WW.CBDD.Bson;
+using ZB.MOM.WW.CBDD.Core;
+using ZB.MOM.WW.CBDD.Core.Collections;
+using ZB.MOM.WW.CBDD.Core.Metadata;
+using ZB.MOM.WW.CBDD.Shared;
+using Xunit;
+
+namespace ZB.MOM.WW.CBDD.Tests;
+
+public class ValueObjectIdTests : IDisposable
+{
+    private readonly string _dbPath = "value_object_ids.db";
+    private readonly Shared.TestDbContext _db;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ValueObjectIdTests"/> class.
+    /// </summary>
+    public ValueObjectIdTests()
+    {
+        if (File.Exists(_dbPath)) File.Delete(_dbPath);
+        _db = new Shared.TestDbContext(_dbPath);
+    }
+
+    /// <summary>
+    /// Executes Should_Support_ValueObject_Id_Conversion.
+    /// </summary>
+    [Fact]
+    public void Should_Support_ValueObject_Id_Conversion()
+    {
+        var order = new Order
+        {
+            Id = new OrderId("ORD-123"),
+            CustomerName = "John Doe"
+        };
+
+        _db.Orders.Insert(order);
+
+        var retrieved = _db.Orders.FindById(new OrderId("ORD-123"));
+
+        retrieved.ShouldNotBeNull();
+        retrieved.Id.Value.ShouldBe("ORD-123");
+        retrieved.CustomerName.ShouldBe("John Doe");
+    }
+
+    /// <summary>
+    /// Executes Dispose.
+    /// </summary>
+    public void Dispose()
+    {
+        _db.Dispose();
+        if (File.Exists(_dbPath)) File.Delete(_dbPath);
+    }
+}