Imports

Section 21.1 - Disjoint-set operations (legacy compatibility path)

Third-edition-numbered compatibility path for the disjoint-set operations. The canonical fourth-edition source is CLRSLean.FourthEdition.Chapter_19.Section_19_1_Disjoint_Set_Operations; this module forwards to it so legacy imports keep working during the compatibility period (see docs/migrations/clrs4.md).