Cushion

The problem

Every competitor solves budgeting by connecting your bank, which forces a backend, an account to attach it to, and an aggregator holding your credentials on their behalf. It also costs the Data Not Collected privacy label. The alternative — manual entry — is only viable if entering a transaction takes three seconds rather than thirty.

What I built

• No per-month budget rows at all. An envelope's balance is derived from the ledger every time, which means carryover is not a feature — it is the absence of a reset. Most envelope apps get this backwards and then have to build carryover on top.
• One invariant holds after every single mutation: total cash equals unassigned plus the sum of every envelope. It is asserted after each operation in the test suite, so an arithmetic bug fails a test rather than quietly losing someone's money.
• Money is integer minor units throughout. Never a Double, never a Float — floating-point money is a rounding error waiting to become a support ticket.
• Statement import reads a CSV or OFX file the bank already offers, parsed on the device with no network request and no credentials anywhere.
• Core logic lives in a Swift package with no UI, no SwiftData and no platform imports, so the 46 tests run from the command line in milliseconds without a simulator.