Class PollingTransport

Interface hosts implement to feed the chart with history + live updates. @rekurt/openkline-core ships PollingTransport and the abstract WebSocketTransport base as concrete starting points, but custom transports are trivially written — the interface has only four methods.

Error handling: both fetchHistory and subscribe should throw (or reject in the async case) on failure; the chart's DataFeed forwards those errors through ChartConfig.onError with a structured ChartError payload.