Imports

Section 21.3 - Disjoint-set forests (legacy compatibility path)

Third-edition-numbered compatibility path for the disjoint-set forests. The canonical fourth-edition source is CLRSLean.FourthEdition.Chapter_19.Section_19_3_Disjoint_Set_Forests; this module forwards to it so legacy imports keep working during the compatibility period (see docs/migrations/clrs4.md).