fix(persistence): preserve Membership duplicate-edge temporal invariant #621

Finding

Membership owner #605 rejects two (member, group, role) assignments whose validity intervals are not strictly Before/After, independent of their weights. Persistence 0010 originally enforced only the same-member/same-role aggregate share budget, so two overlapping rows for the same observed unit, same target group, and same role could persist when their weights summed to at most one. That state is not reconstructable by the Membership owner: it is a duplicate temporal edge, not legitimate multiple membership.

RED → causal repair

• RED 5923151335d440b33df3aab17961451e51c53124 proves same observed unit + same target entity + same role + possible temporal overlap must fail even at 0.5 + 0.5, while strict leave/re-entry, different targets, and different roles remain valid;
• causal repair 98abb27c7c5a29f4ec4ea7502375987cdea84035 adds membership_duplicate_temporal_edge_exists(...) using the same range-native possible-activity envelope as fix(persistence): close unbounded temporal-window bypass in membership share budget #617/fix(persistence): preserve tstzrange endpoint inclusivity in membership overlap admission #618, applies it both to live admission and historical validation, and adds target IDs to the UPDATE trigger surface;
• rollback 81b400b5bb8a869b9622509f90b35f26b85af7e6 removes the new helper; fixture doctoring f2de8acfe9e52ade01b5183c51407f388400b88e preserves a nonempty open-ended leave/re-entry uncertainty window;
• target-only UPDATE acceptance 1c2328259cfb8f507b9b7809ebe565dde809a412 proves changing only target_entity_id cannot create a duplicate edge and that a rejected update leaves the original target unchanged;
• fix(persistence): validate pre-existing membership share budgets on 0010 upgrade #619 upgrade acceptance now also seeds a pre-existing duplicate edge and requires the successor to fail closed before declaring historical state valid.

Different targets with one role remain legitimate multiple membership subject to the exact pointwise #610/#612 share budget. Different roles remain independent. No row merge, target invention, weight normalization, or entity/project identity collapse is used.

Current authority

The surviving vehicle is Draft #615 exact 10b07fcdf47f867e3c7b78ba1b8e8a2a668cc4b3. #621 remains open until current-head PostgreSQL 18 acceptance, coverage/security, qualifying review, prerequisite reconciliation, and normal landing are complete.

Refs #604, #605, #606, #610, #612, #615, #616, #617, #618, #619, #620, #622.