_putenv_s removes a variable for "" and returns EINVAL for NULL, so the
CRT environment cannot represent an explicitly empty value; the boundary
collapsed unset and empty into one state. Read/write through the Win32
environment (SetEnvironmentVariableA/GetEnvironmentVariableA) instead,
keeping a best-effort _putenv_s sync so legacy std::getenv readers stay
coherent, and treat the empty-vs-unset read race fail-closed.
