Keep one unambiguous final Gantt reading

A selected multi-span lane can draw overlapping duration/cost labels, and a later rectangle can paint over an earlier reading. Show one reading for the span ending latest, independent of input order, and paint it after all span rectangles. Full per-span values remain on hover. Tooltip rows also retain distinct React keys when span labels repeat.

Run ID: run_77a0e50b2c0abde48adf048324f3c2e0