Optional ReadonlyonCalled for each lossy-change report a step raises (§80).
Optional ReadonlytargetThe version to upgrade to; SCENE_FORMAT_VERSION by default. A
document already at that version is returned unchanged (validated, in
migrateSceneDocument's case), and a newer one is refused — this build
cannot know what a later format means, and §80 defines upgrades only.
Options for runSceneMigrations and migrateSceneDocument.