feat(session): add explicit leave and immediate slot release

ARCHITECTURE.md

@@ -351,6 +351,13 @@ visible edge and an adjacent server-approved self-cell move across that exact
 edge; it is bounded, replaceable by a newer projection, and never delays the
 received state.
 
+The browser may keep one low-priority heartbeat in flight after a session is
+established. It measures complete round trip with its own monotonic clock; the
+matching acknowledgement contains only a bounded ESP-monotonic handling
+interval. The displayed communication remainder is `max(0, round trip - ESP
+handling)` and is never represented as a synchronized-clock measurement or
+pure Wi-Fi diagnosis.
+
 Player projections are translated only within their received cells so the
 authoritative `self` cell aligns with the responsive local focal point. That
 point is centered horizontally and 30% down the usable portrait map region;