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/Migrations.cs

@@ -0,0 +1,49 @@
+namespace Theriapolis.Core.Persistence.SaveMigrations;
+
+/// <summary>
+/// Registry + chain runner for SaveBody migrations. Phase 4 is the v4 baseline;
+/// older versions are not supported because nothing earlier shipped. As we
+/// bump the schema in Phase 5+, register migrations here.
+///
+/// The registry seeds itself with every built-in migration on first use, so
+/// callers don't need to remember to <see cref="Register"/> them at startup.
+/// </summary>
+public static class Migrations
+{
+    private static readonly List<ISaveMigration> _registry = new();
+    private static bool _seeded;
+
+    public static void Register(ISaveMigration m)
+    {
+        EnsureSeeded();
+        _registry.Add(m);
+    }
+
+    private static void EnsureSeeded()
+    {
+        if (_seeded) return;
+        _seeded = true;
+        // Built-in migrations registered in version order:
+        _registry.Add(new V5ToV6Migration());
+        _registry.Add(new V6ToV7Migration());
+        _registry.Add(new V7ToV8Migration());
+    }
+
+    /// <summary>
+    /// Walk migrations from header.Version up to C.SAVE_SCHEMA_VERSION. Returns
+    /// false if no chain exists; the caller decides whether to hard-block or
+    /// best-effort the load (see SaveCodec docs).
+    /// </summary>
+    public static bool MigrateUp(SaveHeader header, SaveBody body)
+    {
+        EnsureSeeded();
+        while (header.Version < C.SAVE_SCHEMA_VERSION)
+        {
+            var step = _registry.FirstOrDefault(m => m.FromVersion == header.Version);
+            if (step is null) return false;
+            step.Apply(header, body);
+            header.Version = step.ToVersion;
+        }
+        return header.Version == C.SAVE_SCHEMA_VERSION;
+    }
+}