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/Debugger/CtrlDisAsmView.cpp

@@ -209,7 +209,6 @@ CtrlDisAsmView::~CtrlDisAsmView()
 {
 	DeleteObject(font);
 	DeleteObject(boldfont);
-	g_disassemblyManager.clear();
 }
 
 static COLORREF scaleColor(COLORREF color, float factor)