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>

UI/ImDebugger/ImDisasmView.cpp

@@ -38,10 +38,6 @@ ImDisasmView::ImDisasmView() {
 	displaySymbols_ = true;
 }
 
-ImDisasmView::~ImDisasmView() {
-	g_disassemblyManager.clear();
-}
-
 void ImDisasmView::ScanVisibleFunctions() {
 	g_disassemblyManager.analyze(windowStart_, g_disassemblyManager.getNthNextAddress(windowStart_, visibleRows_) - windowStart_);
 }