The type foundation for the leveling engine. The Constraint ADT is a kind-tagged discriminated union — the interchange format between blocks (which emit constraints) and the search (which consumes them); assertNeverConstraint forces exhaustive handling so a new variant cannot be silently dropped. WorkingCalendar is backed by a Uint8Array working-day bitmap plus an Int32Array prefix sum, the shape that makes working-day counting O(1). ResolvedProject owns a default calendar id and a Map of calendars, so per-task and per-resource calendars are first-class, and ResolvedResource carries capacityPerDay. ResolvedTask holds structure only; scheduling state lives on ScheduledTask. Schedule (makespan plus an annotations map for per-block scores and telemetry) is kept separate from Failure, so a failed placement is a distinct value rather than a malformed schedule. WorkUnit is the largest independent work package and is resolved into a by-id registry on ResolvedProject, giving unit definitions a single owner.