Phase 4: Implement SaaS and FP&A model families

[DecisionNerd]

Summary

Build SaaS cohort/unit economics and FP&A budget-vs-actual model families.

Part of #40.

Scope

• Define SaaS revenue, retention, cohort, and unit economics inputs.
• Define FP&A budget, actual, forecast, and variance structures.
• Support diagnostics for missing operating metrics.

Acceptance Criteria

• SaaS and FP&A outputs use shared model-output contracts.
• Tests cover representative cohort, unit economics, and variance calculations.
• APIs expose diagnostics suitable for downstream review.