f51d54a1c1
Cell is the kind-erased hot-path carrier (C7): the value carries no tag, so there are deliberately no tuple-style/tagged constructors (`Cell::I64(..)`). Document on the struct that Scalar is the value to reach for when authoring a param / bind / sweep point / RunManifest field, and that the from_* ctors are the carrier-side entry. Resolves the discoverability papercut without softening the carrier split. closes #100