Tech debt: extend snapshot testing beyond the golden tree

Tracked in `roadmap.md` *Tech debts* as: **"snapshot testing"**

The repo already has one snapshot test that works well: `cmd/scaffold/copy_golden_test.go` (`TestCopy_GoldenSnapshot`) diffs `scaffold.Copy()` output byte-for-byte against `golden/`.

This note is about extending that approach — candidates include the generated OpenAPI document, `wire_gen.go`, and generator output from the not-yet-built `gonext generate *` commands, so template drift is caught the same way scaffold drift already is.

Start with cmd/scaffold/copy_golden_test.go and TestCopy_GoldenSnapshot to understand the existing byte-for-byte golden comparison. Review roadmap.md and identify which generated outputs are ready to snapshot, then add coverage so changes to those outputs are detected; done means the selected snapshots run reliably and catch template drift.

Written by the indexing model from the issue text.