Entity-scoped CAC detail for an analysis period

Customer acquisition cost for one concrete legal entity and an explicit analysis period (default: year to date). The numerator is explicit CAC-tagged sales & marketing spend (GL classification is reconciliation evidence, never a tag); the denominator is canonical new paying customers, available only with continuous subscription history from before the period start. When available is false, reason names why and no numeric zero is fabricated. The top-level available always equals metric.available: a snapshot can compute and still have no ratio (insufficient_inputs), in which case period and breakdown are still returned as evidence while metric.value is null.