E.17.1:17.2 - Migration from local Sigma lists

Legacy MultiViewDescribing uses often publish only one local list of viewpoints. Migration should proceed by:

1. identifying recurring families across several such local lists,
2. publishing those families as explicit bundles,
3. then rewriting the local families to import the new ordinary family designator and declare any subset selection explicitly.

This sequence preserves provenance and avoids pretending that the reusable family had always existed.