Spec: gogobee_expedition_system.md §11. Tier 4–5 zones (Underdark,
Dragon's Lair, Abyss Portal) split into 4 regions each. ExpeditionRegion
struct + per-zone registry; CurrentRegion(e) defaults to Order==1 when
unset; RegionState helpers persist regions_cleared / regions_visited /
base_camps as []string lists. Single-region zones return nil.
Tests cover registry shape (4 per zone, last is IsZoneBoss), lookups,
default current region, and round-trip persistence through DB.
