Usage Billing Regression Lite is a free, local regression fixture for testing usage-billing semantics before invoice changes.

An invoice-total assertion can pass while customer-facing lines are wrong. In the included rounding case, the expected lines are 3¢ and 3¢. A deliberately broken implementation produces 2¢ and 4¢. Both totals are 6¢, so a total-only test misses both errors.

The repository contains two synthetic cases, explicit fictional contracts, event decisions, normalized ledger entries, expected invoice lines, JSON schemas, mutation tests, and a deterministic standard-library Python scorer.

It runs locally with no account, package installation, model call, vendor API, telemetry, or production data. It is intentionally a small, inspectable regression oracle—not a billing engine, certification, or production-validation guarantee.