Add a serialization round trip test

This currently fails because a lot of World still works on references instead of lookups

MultiversalDiplomacyTests/SerializationTest.cs

@@ -0,0 +1,71 @@
+using System.Text.Json;
+
+using MultiversalDiplomacy.Adjudicate;
+using MultiversalDiplomacy.Model;
+
+using NUnit.Framework;
+
+namespace MultiversalDiplomacyTests;
+
+public class SerializationTest
+{
+    [Test]
+    public void SerializeRoundTrip_MDATC_3_A_2()
+    {
+        // Set up MDATC 3.A.2
+        TestCaseBuilder setup = new(World.WithStandardMap(), MovementPhaseAdjudicator.Instance);
+        setup[("a", 0)]
+            .GetReference(out Season s0)
+            ["Germany"]
+                .Army("Mun").MovesTo("Tyr").GetReference(out var mun0)
+            ["Austria"]
+                .Army("Tyr").Holds().GetReference(out var tyr0);
+
+        setup.ValidateOrders();
+        Assert.That(mun0, Is.Valid);
+        Assert.That(tyr0, Is.Valid);
+        setup.AdjudicateOrders();
+        Assert.That(mun0, Is.Repelled);
+        Assert.That(tyr0, Is.NotDislodged);
+        setup.UpdateWorld();
+
+        // Serialize the world
+        JsonSerializerOptions options = new() {
+            WriteIndented = true,
+        };
+        JsonElement serialized = JsonSerializer.SerializeToDocument(setup.World, options).RootElement;
+
+        // Deserialize the world
+        World reserialized = JsonSerializer.Deserialize<World>(serialized)
+            ?? throw new AssertionException("Failed to reserialize world");
+
+        // Resume the test case
+        setup = new(reserialized, MovementPhaseAdjudicator.Instance);
+        setup[("a", 1)]
+            ["Germany"]
+                .Army("Mun").Supports.Army("Mun", season: reserialized.GetSeason("a0")).MoveTo("Tyr").GetReference(out var mun1)
+            ["Austria"]
+                .Army("Tyr").Holds();
+
+        setup.ValidateOrders();
+        Assert.That(mun1, Is.Valid);
+        setup.AdjudicateOrders();
+        Assert.That(mun1, Is.NotCut);
+        Assert.That(mun0, Is.Victorious);
+        Assert.That(tyr0, Is.Dislodged);
+
+        // Confirm that an alternate future is created.
+        World world = setup.UpdateWorld();
+        Season fork = world.GetSeason("b1");
+        Unit tyr1 = world.GetUnitAt("Tyr", fork.Coord);
+        Assert.That(
+            tyr1.Past,
+            Is.EqualTo(mun0.Order.Unit),
+            "Expected A Mun a0 to advance to Tyr b1");
+        Assert.That(
+            world.RetreatingUnits.Count,
+            Is.EqualTo(1),
+            "Expected A Tyr a0 to be in retreat");
+        Assert.That(world.RetreatingUnits.First().Unit, Is.EqualTo(tyr0.Order.Unit));
+    }
+}