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/SaveMigrations/ISaveMigration.cs

@@ -0,0 +1,13 @@
+namespace Theriapolis.Core.Persistence.SaveMigrations;
+
+/// <summary>
+/// Single-step migration from one schema version to the next. Migrations
+/// chain — Migrations.MigrateUp finds a path from header.Version to
+/// C.SAVE_SCHEMA_VERSION and applies each step in order.
+/// </summary>
+public interface ISaveMigration
+{
+    int FromVersion { get; }
+    int ToVersion   { get; }
+    void Apply(SaveHeader header, SaveBody body);
+}