The RunSpec is the one contract the build, jury, integrators, QA, and the receipt share: a versioned spec that freezes the objective, scope, criteria, risks, and proof meaning before agents build. Freeze it before implementation and a changed goal becomes visible instead of convenient.
Planning work cannot pass an execution gate, so Fusion issues an Accepted Receipt instead of a Verified one. The ladder of commitment says exactly how much proof stands behind each artifact.
Mode, profile, and roster are independent controls. Mode decides who drives the workflow. Profile decides whether one builder or a tournament runs. Roster decides how many candidates, judges, and integrators participate and which engines fill those roles.
A Fusion run moves through Discovery, Plan, Build, Review, and Output. Each stage creates a different artifact, owns a different decision, and hands a narrower claim to the next stage.
A Definition of Good is the frozen, testable contract a run must meet: hard criteria that can fail the run, soft criteria that shape it, and a gate a machine can execute.