Fix ReceiveBuffer race condition with dedicated lock object

ProcessReceive() locked on ReceiveBuffer (a byte[]) then reassigned
it inside the lock. Subsequent calls locked on a different object,
breaking mutual exclusion entirely. Replace with a dedicated
readonly _receiveLock object used consistently for all buffer access.

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

Author: wjlafrance