Refactor — one structural experiment

Refactor changes structure while preserving observable behavior. It performs one caller-selected transformation and reports the result.

Prompt

Refactor billing-service/internal/retry/backoff.go: extract the exponential backoff calculation out of RetryRequest into its own function, no other behavior change. Record a baseline, run go test ./internal/retry/... before and after, and report the diff summary, commands, results, and anything not checked.

It's working if

• The report names the preserved behavior and cites go test ./internal/retry/... run both before and after.
• git diff --stat touches only internal/retry/backoff.go, never an unrelated file.
• Golden-output hashes get captured and compared byte-for-byte whenever the changed surface produces output, e.g. sha256sum before and after.
• The report's behavior not checked list is present in the output even when empty, naming any surface the gates skipped.

Procedure