VisibleAxisRange class

Derives the range from the data inside the visible window, so the axis rescales while scrolling instead of growing monotonically with everything ever loaded.

Falls back to the whole-store extent when the window contains no data, so an axis is never left rangeless. (A null AxisRangeContext.visibleX takes the same fallback, but on the render path the y axis always sees a non-null window — even an empty chart synthesises one — so that branch is only reachable from contexts constructed outside layout.)