drpcstream: rename packetQueue to ringBuffer with Enqueue/Dequeue API

Rename the type and its methods to better reflect what the structure
actually is: a bounded ring buffer used as a FIFO queue. Put/Get become
Enqueue/Dequeue, and the stream field is renamed from pbuf to recvQueue
since to the consumer it's just a queue.

This is groundwork for a follow-up that introduces a shared buffer pool.
Done() has a TODO noting it will be removed once Dequeue can transfer
buffer ownership directly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: shubhamdhama