Hold the shutdown lock across all of CPU_Shutdown

It was only held across Memory::Shutdown(), but everything else in there frees
state the debugger UIs read from other threads - kernel objects
(__KernelShutdown), the symbol map, replacements - so a Win32 debugger window
painting while a game is reset could read freed memory. It's recursive, so the
nested acquire in Memory::Shutdown() is unaffected.

No lock-order risk: on the paths where CPU_Shutdown already runs under
g_frameMutex it now takes these in the same frame-then-shutdown order the GUI
side uses, and on the paths where it doesn't (EmuScreen::sendMessage,
ProcessScreenSwitches - both above where NativeFrame takes the guard) it takes
only this one.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01GZq8ZtJmFY7bkX5FVkr3P9

Core/System.cpp

@@ -568,6 +568,12 @@ static bool CPU_Init(FileLoader *fileLoader, IdentifiedFileType type, std::strin
 }
 
 void CPU_Shutdown(bool success) {
+	// Held across the whole teardown, not just Memory::Shutdown() further down. Everything below
+	// frees state the debugger UIs read from other threads - kernel objects, the symbol map, the
+	// memory map - and this is the lock they take to be sure none of it goes away mid-read. See
+	// Memory::Lock(); it's recursive, so the nested acquire in Memory::Shutdown() is fine.
+	Memory::MemoryInitedLock coreLock = Memory::Lock();
+
 	UninstallExceptionHandler();
 
 	GPURecord::Replay_Unload();