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>

Core/Debugger/WebSocket/SteppingSubscriber.cpp

@@ -33,9 +33,6 @@ struct WebSocketSteppingState : public DebuggerSubscriber {
 	WebSocketSteppingState() {
 		g_disassemblyManager.setCpu(currentDebugMIPS);
 	}
-	~WebSocketSteppingState() {
-		g_disassemblyManager.clear();
-	}
 
 	void Into(DebuggerRequest &req);
 	void Over(DebuggerRequest &req);