One shared CURRENT_VERSION across the three top-level serialised
payloads (VideoPredictions, JobResults, BenchmarkResult), with
per-schema registries populated via register_*_migration(from_version)
decorators. FutureSchemaError and MigrationNotFoundError surface bad
chains clearly. CURRENT_VERSION=2 with v1→v2 migrations registered
that add an optional provenance field to the payload dicts.
Tested standalone; io.py is wired through the migrator in a follow-up
commit that introduces the Provenance schema those migrations target.
