fix(flow): re-arm multi-source or_ listeners across router-driven cycles

The previous discard-after-body approach cleared the gate mid-wave, so
a slow parallel @start finishing after the listener body could re-fire
the same multi-source or_ listener. Re-arm only when a router emits a
signal that matches the listener's condition; parallel @start paths
never reach that branch and the race gate keeps protecting them.

Closes #5972