Lightweight presence heartbeat polled by the coach Active Session screen (~4s cadence) while the session is in_progress. Refreshes the coach's last-seen timestamp in sessions.coach_presence and, if the coach is returning after the session was unattended, closes the open coach_absence_log interval (AC9/AC11).

Drives the participant "coach has temporarily stepped away" notice: when every assigned coach stops heartbeating (tab/window closed, crash, or a deliberate Leave), the freshness window lapses and participants see the notice within one poll. No-op when the session is not in_progress.