Build the PrimitiveHard
Largest Rectangle in Histogram
Write `largest_rectangle_area(heights)` returning the area of the largest rectangle that fits under the histogram.
largest_rectangle_area([2, 1, 5, 6, 2, 3]) -> 10 (heights 5 and 6, width 2)
Every bar is the limiting height of some maximal rectangle. Keep a stack of increasing bars; when a shorter one arrives, pop and settle each taller bar's rectangle. The part people get wrong is the width: a popped bar's rectangle starts where the *last bar it displaced* started, not at its own index.
What to expect: A timer starts when you begin. Edit the starter code, run it against the test suite as many times as you like, then finish when you're done. The reference solution and interviewer follow-up questions unlock only after you finish.