Update test schema with a pure-source (non-indexed) type feeding a top-level sourced_from field

[ellisandrews-toast]

Part of #1273. The nested sourced_from case is covered end-to-end by the CoachProfile/GeneralManagerProfile types, but we had no coverage proving a top-level sourced_from field can be fed by a pure-source (non-indexed) type.

This adds ComponentDesign — a small pure-source type (no t.index) whose events feed a new top-level Component.designer_name field via an indexing_only: true relationship — plus acceptance coverage in top_level_multi_source_indexing_spec.rb.

No library changes were needed — the prior PRs in this stack already handle everything, so this is purely test schema + factory + spec additions and the resulting artifact churn.