fix: dispatch mouseLeave to the tracked widget on chart leave

[NemeZZiZZ]

Problem

Event.mouseLeaveEvent (the chart-level leave, fired from the container's mouseleave) clears the crosshair but never tells the tracked widget that the pointer left:

mouseLeaveEvent(): boolean {
  this._chart.getChartStore().setCrosshair()
  return true
}

Meanwhile mouseMoveEvent maintains _mouseMoveTriggerWidgetInfo and dispatches mouseEnterEvent/mouseLeaveEvent to widgets as the pointer moves between them (Event.ts:352-356) — but on leaving the whole chart, that machinery is skipped.

Impact

SeparatorWidget._mouseLeaveEvent is the only place that restores the separator's background to transparent. Hover a pane separator and move the cursor out of the chart: the separator stays highlighted. _mouseMoveTriggerWidgetInfo also goes stale, so re-entering the same widget does not re-fire mouseEnterEvent (the widget still believes the pointer is inside).

Fix

Before clearing the crosshair, dispatch mouseLeaveEvent to the tracked widget (with a widget-local event, via _makeWidgetEvent) and reset _mouseMoveTriggerWidgetInfo — mirroring the move path. The method now accepts the compat event EventHandler already passes to it.