Shared opportunity constraints and transport-policy rankings: Interval stability and an exact three-destination reversal

This technical note compares the minimum feasible dissimilarity under two fixed accessibility policies evaluated in the same opportunity world. The model has two fixed population groups, one origin per group, continuous allocations, fixed fully used population and destination margins, fixed closed linear policy restrictions, and a shared closed linear opportunity restriction. Both policies are feasible in every admitted world. For two destinations, an interval-distance identity excludes strict ranking reversal across opportunity worlds. An explicit three-destination family attains reversal, with an exact eligibility threshold, optimal allocations, and sharp common-world contrast range. Thus three destinations are minimal within the stated class. A rational example has common-world contrast range [-1/10, 1/10], while the marginal rectangle gives [-2/5, 2/5]. Appendix A records predecessor comparisons and the actual source-reading scope. Version v1 is the first public technical-note release, dated 27 October 2026.