winreg is a native extension exposing the Win32 registry through a typed, hard-to-misuse Ruby API: strict typed readers and writers for REG_SZ, REG_EXPAND_SZ (never auto-expanded), REG_MULTI_SZ (correct double-NUL wire format), REG_DWORD/REG_QWORD (range-checked), and REG_BINARY, with raw escape hatches for adversarial data; default-value access; 32/64-bit registry views as a first-class option applied consistently to child operations; least-privilege KEY_READ defaults; and RegNotifyChangeKeyValue change watching (thread-agnostic registrations, rearm-before-deliver) that blocks cooperatively under a Fiber scheduler and releases the GVL standalone. Windows MSVC (mswin) Ruby only.