Fix `mainmemory` lib keeping reference to unloaded core's memdomain

src/BizHawk.Client.Common/lua/LuaHelperLibs/MainMemoryLuaLibrary.cs

@@ -25,6 +25,12 @@ namespace BizHawk.Client.Common
 
 		private string MainMemName => _mainMemName ??= APIs.Memory.MainMemoryName;
 
+		public override void Restarted()
+		{
+			_mainMemDomain = null;
+			_mainMemName = null;
+		}
+
 		[LuaMethodExample("local stmaiget = mainmemory.getname( );")]
 		[LuaMethod("getname", "returns the name of the domain defined as main memory for the given core")]
 		public string GetName()

src/BizHawk.Client.Common/lua/LuaLibraryBase.cs

@@ -1,6 +1,8 @@
 using System;
 using System.Threading;
 
+using BizHawk.Emulation.Common;
+
 namespace BizHawk.Client.Common
 {
 	public abstract class LuaLibraryBase
@@ -40,6 +42,9 @@ namespace BizHawk.Client.Common
 			}
 		}
 
+		/// <remarks>for implementors to reset any fields whose value depends on <see cref="APIs"/> or a <see cref="IEmulatorService">service</see></remarks>
+		public virtual void Restarted() {}
+
 		/// <exception cref="InvalidOperationException">attempted to have Lua running in two host threads at once</exception>
 		public static void SetCurrentThread(LuaFile luaFile)
 		{

src/BizHawk.Client.EmuHawk/tools/Lua/LuaLibraries.cs

@@ -171,6 +171,7 @@ namespace BizHawk.Client.EmuHawk
 			{
 				lib.APIs = _apiContainer;
 				ServiceInjector.UpdateServices(newServiceProvider, lib);
+				lib.Restarted();
 			}
 		}