How do I build the horizontal reel above the reels that shows which symbols pay this round?

Use HorizontalReelBuilder — it builds a HorizontalReel, a single one-row reel oriented sideways. Register your symbols and give it a ticker, exactly like ReelSetBuilder. It follows the same spin contract as ReelSet: spin() starts it and returns a promise, setResult([{ visible: ids }]) takes the same ColumnTarget[] (this reel is one column, so pass one entry with visibleCount ids) and triggers the stop, and the spin() promise resolves with the engine's SpinResult. Add reel.container to the stage above your ReelSet.