feat: NativePipe cppgc object inheriting from LibUvStreamWrap
NativePipe wraps uv_pipe_t and inherits readStart/readStop/writeBuffer/
shutdown/close/ref/unref from LibUvStreamWrap (same pattern as TTY).
Pipe.open(fd) creates a NativePipe, opens the fd via uv_pipe_open,
and delegates readStart/readStop/writeBuffer/ref/unref to it.
main_read and main_write pipe_open_fd tests pass.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
