fix(persistence): enforce membership share budgets pointwise across time #620

Finding

The predecessor 0010_membership_same_role_share_budget trigger summed every existing membership whose possible-activity range overlapped the candidate range. That is stricter than the Membership owner invariant because different existing spells may overlap a spanning candidate at different times without ever overlapping one another.

Counterexample:

• A: weight 0.6, active [2026-02-01, 2026-02-10];
• B: weight 0.6, active [2026-02-20, 2026-02-28];
• candidate C: weight 0.4, active [2026-02-05, 2026-02-25].

At every event time the true same-role total is at most 1.0 (A+C early, B+C late), but pairwise-candidate aggregation produces 1.6 and rejects a scientifically valid time-varying history.

RED → causal repair

• behavioral RED 6eaa5dc2caf601b65112996a436f37e455a2916e adds the disjoint-existing/spanning-candidate positive case plus a genuine common-time .4 + .4 + .3 negative control;
• causal repair aa43c6520f71744504c703bd853a97a53571341f replaces pairwise-overlap SUM with an event-state sweep over exact finite boundary points, right-open states, and the lower-unbounded initial state;
• exact represented-binary64 numerators from scientific(membership): make same-role share-budget admission exact at the binary64 boundary #612 are retained; PostgreSQL tstzrange open/closed and unbounded semantics from fix(persistence): close unbounded temporal-window bypass in membership share budget #617/fix(persistence): preserve tstzrange endpoint inclusivity in membership overlap admission #618 remain authoritative;
• rollback helper repair 75d6f65de7c4dd3bcc8f67d3d747a69a7a033e95 keeps the successor reversible.

No timestamp epsilon, tolerance, normalization, decimal-only aggregate, or scalar temporal collapse is used.

Current authority

The surviving vehicle is Draft #615, currently exact 10b07fcdf47f867e3c7b78ba1b8e8a2a668cc4b3, which also carries #619/#621/#622 refinements. #620 remains open until that exact lineage has hosted PostgreSQL 18 acceptance, current-head security/coverage evidence, independent review, prerequisite reconciliation, and normal landing.

Refs #604, #605, #610, #612, #615, #616, #617, #618, #619, #621, #622.