controller property

Optional public handle for driving and observing zoom from outside the widget (#76) — e.g. a host's own zoom toolbar. When supplied it attaches to the planner's internal zoom/scroll controller for the life of this widget (and across a controller swap); when null, zoom is driven only by the built-in controls, pinch and Ctrl+wheel, exactly as before. Pair it with config.showZoomControls: false to replace the on-canvas buttons.