Task core-engine-value-constructors (wave-3 pre-work). All four value
types are #[non_exhaustive] (ADR-017 §3), so downstream engine crates
cannot struct-literal-construct them (E0639). Give engines a sanctioned
construction path without weakening the consumer posture: pub
#[doc(hidden)] full-field constructors (Job::from_row, StreamEvent::
from_row, Schedule::new, Wake::new), each doc-commented as engine-
construction-only — not contract surface, not covered by ADR-017's
semver-minor field-addition promise; a field addition changes the
signature and is a lockstep-duty event (ADR-017 §5). Core tests now
construct through the new constructors; no behavior change.
glm-5.3-flash