Implement native layout-3 save restoration

engine/Age.Engine/Hosting/IScriptProvider.cs

@@ -8,6 +8,9 @@ public interface IScriptProvider
     /// <summary>The script for this id, or null if the id maps to no known script.</summary>
     Script? GetById(long id);
 
+    /// <summary>Resolve a fixed engine callback filename such as CALLBACK_LOAD.BIN.</summary>
+    Script? GetByName(string name) => null;
+
     /// <summary>Selectors of currently mounted append catalogs. Opcode 0x143 scans these in
     /// ascending order and executes record zero from each catalog.</summary>
     IReadOnlyList<int> MountedAppendSelectors => Array.Empty<int>();