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/WebSocket/MemorySubscriber.cpp

@@ -63,7 +63,7 @@ struct AutoDisabledReplacements {
 // by the time this is called, so nothing else can be concurrently executing MIPS code or touching the
 // JIT's emuhack ops on this thread while we hold onto them below.
 //
-// Deliberately does NOT take a Memory::MemoryInitedLock: memory teardown only ever happens on the
+// Deliberately does NOT take a CoreShutdownLock: memory teardown only ever happens on the
 // CPU thread too, so there's nothing to guard against, and taking it here deadlocked against the
 // Win32 debugger's paint handlers. See the lock ordering section in AGENTS.md.
 //