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

Windows/Debugger/DumpMemoryWindow.cpp

@@ -88,7 +88,7 @@ INT_PTR CALLBACK DumpMemoryWindow::dlgFunc(HWND hwnd, UINT iMsg, WPARAM wParam,
 				// queued callback.
 				enum class Outcome { NotInited, OpenFailed, Success } outcome = Outcome::NotInited;
 				Core_RunOnCPUThread([&] {
-					Memory::MemoryInitedLock memLock = Memory::Lock();
+					CoreShutdownLock coreLock = Core_LockAgainstShutdown();
 					if (!PSP_IsInited())
 						return;