The _send_seq += 1 approach had a race condition: two threads calling
send() simultaneously could load the same counter value, producing two
queue items with identical (priority, seq) tuples. heapq then compares
the third element (protobuf message), which crashes with TypeError.

Replace with itertools.count() whose next() is GIL-atomic in CPython —
each call is a single C-level operation that cannot be interrupted mid-
increment. Also fix the legacy fallback path in LiveNodeRecord.send_message
and remove the duplicate 'import queue' in node_registry.py.

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