graph, store: Reject grafts below the base's earliest block (#6610)

Mirrors the guard added for `graphman copy` in #6384. Without this,
grafting at a block below `base.earliest_block_number` silently produces
a destination missing all pre-graft mutable entity versions, and (when
the base has dynamic data sources) handlers fail with
`unexpected null in handler` because `data_sources$` registrations
survive pruning while the entities they reference do not.