Clarify joinSchema relation to a lineage's schemas: & or must()?

[sdboyer]

Currently, #Lineage.joinSchema is unified with all schemas in a lineage. Sometimes that's desirable. Other times, i think what we want is really something more like a must()/constraint.

These should probably be teased out into two distinct capabilities.

[frossano-grafana]

make users write that they don't have default panel options instead assuming that when there is nothing the defaul is null
the net effect is the same, the change is in ergonomic of using Thema