Initial commit: Theriapolis baseline at port/godot branch point

Captures the pre-Godot-port state of the codebase. This is the rollback
anchor for the Godot port (M0 of theriapolis-rpg-implementation-plan-godot-port.md).
All Phase 0 through Phase 6.5 work is included; Phase 7 is in flight.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Theriapolis.Tests/Persistence/V4ToV5MigrationTests.cs

@@ -0,0 +1,46 @@
+using Theriapolis.Core;
+using Theriapolis.Core.Persistence;
+using Xunit;
+
+namespace Theriapolis.Tests.Persistence;
+
+/// <summary>
+/// Phase 5 M2 refuses Phase-4 saves rather than auto-instantiating a default
+/// Character. Verified via <see cref="SaveCodec.IsCompatible"/> +
+/// <see cref="SaveCodec.IncompatibilityReason"/>.
+/// </summary>
+public sealed class V4ToV5MigrationTests
+{
+    [Fact]
+    public void V4Header_IsRejectedAsIncompatible()
+    {
+        var header = new SaveHeader { Version = 4 };
+        Assert.False(SaveCodec.IsCompatible(header));
+        Assert.NotEmpty(SaveCodec.IncompatibilityReason(header));
+    }
+
+    [Fact]
+    public void V5Header_IsCompatible()
+    {
+        var header = new SaveHeader { Version = 5 };
+        Assert.True(SaveCodec.IsCompatible(header));
+        Assert.Empty(SaveCodec.IncompatibilityReason(header));
+    }
+
+    [Fact]
+    public void NewSaveHeader_AutoSetsCurrentSchemaVersion()
+    {
+        var header = new SaveHeader();
+        Assert.Equal(C.SAVE_SCHEMA_VERSION, header.Version);
+        Assert.True(header.Version >= C.SAVE_SCHEMA_MIN_VERSION);
+    }
+
+    [Fact]
+    public void IncompatibilityReason_MentionsVersionInformation()
+    {
+        var header = new SaveHeader { Version = 3 };
+        var reason = SaveCodec.IncompatibilityReason(header);
+        Assert.Contains("v3", reason);
+        Assert.Contains("v" + C.SAVE_SCHEMA_MIN_VERSION, reason);
+    }
+}