fix: avoid loader-lock work in DllMain detach

[H-Chris233]

Problem

DllMain's DLL_PROCESS_DETACH path called SteamUI::CoreUnhook() and SteamClient::CoreUnhook(), which may perform LoadLibrary/FreeLibrary under the loader lock. This is unsafe during process termination and can cause deadlocks or undefined behavior.

Additionally, nothing prevented an external FreeLibrary from unloading the DLL while hooks and worker threads still reference its code.

Changes

1. Pin the module on DLL_PROCESS_ATTACH via GetModuleHandleExA(GET_MODULE_HANDLE_EX_FLAG_PIN) so the DLL cannot be unmapped while still in use.

2. Guard DETACH work: during process termination (pvReserved != nullptr), only call FileWatcher::Stop() to avoid std::thread's joinable-guard std::terminate. Skip CoreUnhook entirely since it may acquire the loader lock.

Testing

• Verified DLL attaches/detaches cleanly in Steam processes.
• FileWatcher thread joins properly during exit (no more std::terminate crashes).