End-to-end SLO with dependency attribution

Design SLO monitoring for a user-facing service whose request depends on 6 internal microservices and 2 external vendors (a payment processor and an email provider). The product team owns a 99.9% end-to-end SLO but each dependency has its own SLO and its own owner. You need to (a) measure the true user-experienced SLO, (b) attribute budget burn to the dependency that caused it, and (c) catch failures even during zero-traffic periods. Design the SLI measurement, dependency attribution, and synthetic coverage.