Reduced abelian p-groups: legacy compatibility import

Historically the core algebra for reduced abelian p-groups lived in this file. It now re-exports the reorganized PGroups layer:

• Lib.PGroups.Subgroups
• Lib.PGroups.UlmSubgroups
• Lib.PGroups.Heights

Existing imports of Lib.PGroups.Defs therefore continue to work while the library is migrated to the new layout.