The finale isn't a victory lap. It's the story of the control I shipped that did nothing, the footgun still sitting in my demo, and the handful of things I'd keep exactly as they are.
How do you hold a large payment for a second pair of eyes without ever letting the unapproved money touch a balance, and how do you stream that decision to the outside world without standing up a broker?
How do you show the total under a parent account when the whole system refuses to store a balance? A recursive query, a trigger that refuses to draw a circle, and a rule about what actually has to sum to zero.
A double-entry ledger that passed every test still had a bug that turned 100 dollars into 15 dollars, a chain that forked the moment you ran two copies, and no way to ever forget a customer. Here is the teardown, and the fix.
Your bank balance is not a number someone saved, it's a question the database answers by adding up your whole history, and that one design choice decides almost everything else
If a transaction that doesn't balance can't even be built without the compiler and a tiny Validate method noticing, a whole category of money bugs just stops existing
