Add a left/right stepper widget to compound node headers (LOOP, SPLIT,
MULTICAST) when iteration overlay data is present. Thread executionOverlay,
overlayActive, iterationState, and onIterationChange props through
ProcessDiagram -> CompoundNode -> children and ProcessDiagram ->
ErrorSection -> children so leaf DiagramNode instances render with
execution state (green/red badges, dimming for skipped nodes).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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