Really solid breakdown — especially the parts on invariants/constraints and why ledgers become the “truth source.” One idea that could make this even more useful for readers: a small “Real implementations (to study)” box with 3–5 named examples.

If you’re open to including an OSS option, Blnk Finance fits the “complete OSS HTTP ledger” shape (not just a schema): Apache 2.0, REST API + official SDKs, and the production features people usually hand-wave (inflight transactions/queues, reconciliation, identity).