Reference-set currency: the two properties 126 and 127 introduced are untested #50

Landing the run-model stack ported test_two_campaign_runs_each_hold_a_current_refimage_for_one_identity to reference-set granularity, which is coexistence ACROSS sets. Two properties the migrations introduced still have no test. (1) THE WITHIN-ONE-SET COLLISION: refimages_vbest_current_per_set_unique ON (field, fid, ppid, reference_set_id) WHERE vbest IN (1, 2) must REFUSE a second current reference for one identity group inside a single set — the uniqueness half of the same index whose coexistence half is now covered. The ported test deliberately gives each run its own set, so it never exercises the refusal. (2) THE SET-SCOPED DEMOTION GUARD: 127's header calls this "the correctness heart of the change" — demotion must not reach across sets, so registering into one set leaves another set's current row alone. Nothing asserts either. Both are testing debt from a stabilisation pass that was scoped to porting existing properties only (Ben, 2026-09-14 18:30: "no feature or restructuring inside a stabilisation pass"), to become a small brief on the landed smdc now that it is green. Verified 2026-09-14 against branch tip 102f0f2c: actions.create_reference_set and a 19-parameter actions.create_run with reference_set_id both already exist, so the fuller tests need no new Python surface — the earlier "wrapper gap" reading came from a stale local checkout and is NOT part of this row