When direct P2P calls show 100% datagram drops, we need to know
WHY send_media() fails. This commit adds:
- Remote address + stable_id logging on A-role accept and D-role
dial success (dual_path.rs) — tells us which candidate won
- Remote address + max_datagram_size on engine transport init —
verifies datagrams are negotiated
- last_send_err in send heartbeat — captures the actual error
from send_datagram() failures
- QuinnTransport::remote_address() helper
Also fixes UI badge: was looking for wrong event name
("dual_path_race_won" → "path_negotiated").
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
40955bd11c