Multi-file refactor with verification
Below is a 6-file Go service in `internal/payment/`. Refactor it so the `Charge` interface lives in its own file under `internal/payment/iface/`, and so the test fakes move to `internal/payment/iftest/`. Update all call sites. Then write a shell command sequence (no prose) that lints, builds, and runs the test suite to verify your change. [6 files inlined...]
Model produces full diffs for every file plus a verification block. GPT-5.5 and Opus 4.7 both reliably keep import paths consistent across files; GPT-5.3-Codex tends to win on the verification block's terseness.