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/System.cpp

@@ -50,6 +50,7 @@
 #include "Core/MIPS/MIPS.h"
 #include "Core/MIPS/MIPSAnalyst.h"
 #include "Core/MIPS/MIPSVFPUUtils.h"
+#include "Core/Debugger/DisassemblyManager.h"
 #include "Core/Debugger/LineInfo.h"
 #include "Core/Debugger/SymbolMap.h"
 #include "Core/System.h"
@@ -616,6 +617,10 @@ void CPU_Shutdown(bool success) {
 
 	pspFileSystem.Shutdown();  // This unmounts all filesystems.
 
+	// Everything the disassembly cache describes - emulated memory and the symbol map - is about
+	// to go away, so drop it here rather than leaving it to whichever debugger UI closes last.
+	ClearDisassemblyCache();
+
 	mipsr4k.Shutdown();
 	Memory::Shutdown();
 	HLEPlugins::Shutdown();