Address PR review: collab guest reconciles a state frame with
isStreaming === false by stopping the loader when the host's
agent_end never reached us (typically across a reconnect). With the
new active-time meter, the same path also has to call markActivityEnd
or the per-session window stays open and time_spent ticks forever.
Extract the close logic as reconcileGuestIdleHostState so a unit test
can exercise it directly without bringing up the full host/relay
welcome train.
roboomp
9653e38526