fix(timer): correctly handle multi-instance teardown

lua/jasper/timer.lua

@@ -328,8 +328,25 @@ function M.teardown()
     state.uv_timer:close()
     state.uv_timer = nil
   end
+
+  if not (state.running and state.timer_id) then
+    cleanup_activity_file()
+    return
+  end
+
+  -- Remove our own timestamp from the shared file (only if we were the last
+  -- writer). After this, other_instance_active() reflects *other* instances only.
   cleanup_activity_file()
-  if state.running and state.timer_id then
+
+  if other_instance_active() then
+    -- A different session is still actively using this timer: leave it running.
+    notify("Another instance is active — timer left running on exit")
+  else
+    -- No currently-active other instance. However, there may be a surviving
+    -- idle instance (e.g. instance A whose watchdog re-armed because this
+    -- instance was writing activity). Write the "0" sentinel so that instance
+    -- detects the stop on its next user activity and resumes the timer.
+    mark_timer_paused()
     local ok, err = api.stop_timer(state.token, state.timer_id)
     if ok then
       state.running = false