The Burl-Holds of the Ossenwald Greatoak

The Thornwarden Guild posts a commission for climbers bold enough to scale the Ossenwald Greatoak, a tree so vast its crown is lost in cloud. The trunk is smooth save for burls, swollen knots in the bark that serve as holds. The Guild's surveyors have measured the height of every burl on the climbing line. They need to know how high a climber of a given reach can get before the bark leaves them stranded.

The rules of the climb:

• The climber starts on the forest floor, at height 0.
• The burls are taken strictly in the order given, which is non-decreasing by height.
• From their current height, the climber may move onto the next burl only if that burl's height minus the current height is at most reach. If they do, their current height becomes that burl's height.
• At the first burl whose gap is greater than reach, the climb ends. No later burl is considered.

Write climbHeight(holds, reach), where:

• holds is a list of non-negative integers in non-decreasing order, giving the burls' heights in feet. It may be empty.
• reach is a non-negative integer, in feet.

Return an integer: the climber's height when the climb ends. If every burl is climbed, return the height of the last burl. If no burl is climbed, return 0.