Register a KPN event handler in both scene_analyze and scene_preview:
- Overflow events accumulate per-node dropped-frame counts, printed on exit.
- A Closed event from any node other than result_sink at EOF means a stage
died; trip an atomic so the main loop bails out instead of hanging on
'done' forever, and exit non-zero.
Bumps external/KPN to the commit that exposes set_event_handler / NodeEvent.