Cost Line
Identifier for a single line item produced by a CostFormulation. Used as a key in CostCalculator.lineResponses and as the index into CostFormulation.byLineResponse for per-line rollup queries.
The complete list of v1 line items is exposed via all so formulations can iterate over every line when building rollup aggregates. v2 line items (introduced by Phase 5 — backorder, stockout, lost-sale, unit-shortage) are included here from the start so the formulation infrastructure does not need to grow as those calculators land.