When dragging a marker in a multi-client PubSub setup, the server echoes
position updates back to the originating client. The data-effect would
then call setLngLat with the stale echoed position, snapping the marker
back while the user had already dragged further.
Add a per-marker _dragging flag: set on dragstart, cleared after the
final dragend writeback. The signal→position effect skips setLngLat
(and rotation) while the flag is true.
