Retracted: Retract v0.1.0 through v0.1.3: v0.1.0–v0.1.2 rewrote the journal
(async RunStep + dropped seq). v0.1.3 is the last release before
typed step input and WithStepVersion. Start from v0.1.4.
Package main demonstrates the functional / closure style of durable-go.
A payment task runs three steps: validate, charge, and send receipt.
If the process crashes after "charge-stripe" succeeds, re-running with the
same taskID replays the cached result and only executes "send-receipt".