Misc shutdown fixes

* Destroy the ImGui debugger explicitly, not at static destruction time

* Windows: destroy the debugger windows before NativeShutdown()

* Clear the disassembly cache where it goes stale, not in view destructors
  (behind a no-op stub for libretro, which doesn't build DisassemblyManager.cpp)

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Windows/main.cpp

@@ -1312,6 +1312,11 @@ int WINAPI WinMain(HINSTANCE _hInstance, HINSTANCE hPrevInstance, LPSTR szCmdLin
 	_dbg_assert_(mainThread.joinable());
 	mainThread.join();
 
+	// The debugger windows reach into core state as they go away (CtrlDisAsmView touches
+	// g_disassemblyManager, for example), so tear them down while the core is still around.
+	MainWindow::DestroyDebugWindows();
+	DialogManager::DestroyAll();
+
 	// It's safe to call NativeShutdown, we've joined the main thread.
 	NativeShutdown();
 
@@ -1319,9 +1324,6 @@ int WINAPI WinMain(HINSTANCE _hInstance, HINSTANCE hPrevInstance, LPSTR szCmdLin
 
 	// g_InputManager.StopPolling() is called in WM_DESTROY
 
-	MainWindow::DestroyDebugWindows();
-	DialogManager::DestroyAll();
-
 	TimeShutdown();
 
 	UnRegisterCMPTMFApis();