Send a DM to any contact, measure round-trip time. Both devices
exchange their GPS coordinates; the result shows RTT, distance and
cardinal bearing when both have a fix. The app also auto-replies to
incoming pings so two devices running Ping can time each other.
Harness additions: send_dm mock, wada.geo (haversine), deliver_dm
helper, and 6 scenarios (tdeck, v4, no_contacts, round_trip,
auto_reply, cost) — all pass.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
