Conveyor Item Moved
@Serializable
@SerialName(value = "ConveyorItemMoved" )
A conveyed item advanced to cell cellIndex of its conveyor (the single ConveyorRequest.moveForwardOneCell chokepoint). The renderer maps the cell index to a world position via the conveyor's ConveyorDefined anchors and interpolates between consecutive samples — the conveyor analog of the agent position stream (8G.5). Captures accumulation and blocking for free: when an item cannot advance, no event fires and it holds on the belt.