Rename Memory::Lock() to Core_LockAgainstShutdown(), move it to Core

It stopped being about memory when CPU_Shutdown started holding it across the
whole teardown - it's what keeps kernel objects, the symbol map and the memory
map from being freed while another thread reads them. The old name invited the
reading that it locks memory *access*, which it has never done.

Memory::Reinit() now holds it across both halves rather than relying on
Memory::Shutdown()'s own acquire: between Shutdown() and Init() there is no
memory map at all, and a reader could slip into that gap.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GZq8ZtJmFY7bkX5FVkr3P9

Core/Debugger/DisassemblyManager.cpp

@@ -28,6 +28,7 @@
 #include "Common/Log.h"
 #include "Common/StringUtils.h"
 #include "Common/Math/math_util.h"
+#include "Core/Core.h"
 #include "Core/MemMap.h"
 #include "Core/System.h"
 #include "Core/MIPS/MIPSDebugInterface.h"
@@ -992,7 +993,7 @@ bool GetDisasmAddressText(u32 address, char *dest, size_t bufSize, bool abbrevia
 
 // Utilify function from the old debugger.
 std::string DisassembleRange(u32 start, u32 size, bool displaySymbols, MIPSDebugInterface *debugger) {
-	auto memLock = Memory::Lock();
+	CoreShutdownLock coreLock = Core_LockAgainstShutdown();
 	std::string result;
 
 	// gather all branch targets without labels