Continue PR #207 from cce45bd2bb3a9c1715ef2d4ad6269481dda5ed42. Input comparison: Fable's PR #208 at a343fed9b396bfa32b7f415576842e8c051458b5. Leave both PRs open. The 615-unit bundle is completed work and must not be regrouped or checked again in this continuation.
Independently verify the coordinator's rational B/1387 on every attainable halving-fold column at N=10000,y=100, including empty-source columns. Enclose its logarithmic cost and prove the upper-bound signs. Identify exactly the nonnegative coefficient family that cannot attain the saved 132.729535 prefix-exchange gain. Check the descending signed-fold recurrence and completeness under attainable-prefix assumptions, including prefix reconstruction. This is triangular algebra, not a new arithmetic estimate or a rule selecting feasible signed coefficients.
Ownership: new note, exact checker, input, output and computation manifest in this hunt, one focused test, generated CONTEXT.md. Fable owns converting the existing witness and its construction search. No new optimizer, matching-optimum calculation, witness-coordinate search, larger-N batch or background research job. One numerical thread; fixed bounded checks only. Preserve all prior outputs. Commit and push to #207 without merging.