Generates a single Ecto migration file for every entity in a Caravela.Schema.Domain.

Returns {path, source} — the caller writes the file. The migration file name is timestamped so subsequent runs produce a distinct file.

Deterministic output

By default the migration's filename prefix is now() in UTC. For snapshot tests, demos, or any scenario where you want byte-stable output across runs, pass :timestamp:

Caravela.Gen.Migration.render(domain, timestamp: "00000000000000")

Without this pin, every render produces a new path and snapshot diffs always appear as "add + remove".