Re-cut of the never-promoted v0.12.6 candidate plus 11 production-facing
fixes merged since (#546 #578 #579 #591 #628 #655 #656 #657 #659 #660 +
witness-baseline #658). :v012 users on 0.12.4 receive the full v0.12.4...v0.12.7
batch on promote.