Cap Stream receive buffer so incomplete frames cannot grow until OOM #8

lib/msgpack.js concatenates self.buf on every data with allocUnsafe(self.buf.length + d.length) and no max. Incomplete frames (kScanContinue) keep the buffer. SEND_QUEUE_CAP bounds send, not receive. close/end/error do not clear self.buf.

Oversized headers already throw and drop the buffer. Incomplete frames under ScanOne still grow.

Reject (and reset the socket) when self.buf.length + d.length would exceed a documented MAX_STREAM_BYTES before allocate.