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.Core/Persistence/IPersistable.cs

@@ -0,0 +1,14 @@
+namespace Theriapolis.Core.Persistence;
+
+/// <summary>
+/// Modules that own save-worthy state implement this interface.
+/// CaptureState produces a serializable snapshot; RestoreState applies one
+/// over the live module. Phase 5/6 add new persistables (faction/quest/rep)
+/// without churning the SaveBody schema, by adding new IPersistable types
+/// and one new field on SaveBody.
+/// </summary>
+public interface IPersistable<TState>
+{
+    TState CaptureState();
+    void   RestoreState(TState state);
+}