Adds greeting_trigger_node to saltybot_social:
- Subscribes to /social/faces/detected (FaceDetectionArray) for face arrivals
- Subscribes to /social/person_states (PersonStateArray) to cache face_id→distance
- Fires greeting when face_id is within proximity_m (default 2m) and
not in per-face_id cooldown window (default 300s)
- Publishes JSON on /saltybot/greeting_trigger:
{face_id, person_name, distance_m, ts}
- unknown_distance param controls assumed distance for faces with no PersonState yet
- Thread-safe distance cache and greeted map
- 50/50 tests passing
Closes#270
