SearchResult.attachments: aggregate downloadable-files key unmodeled

[jeremy]

Summary

SearchResult responses carry a generic attachments key — the aggregate
list of downloadable files on the underlying recording — that the SDK does not
model. It is emitted at app/views/api/searches/show.json.jbuilder:25 and is a
SearchResult field, not a generic Recording field.

Distinct from the rich-text companion arrays

This is not a rich-text *_attachments array. #408 completed coverage of
the rich-text companion arrays (content_attachments / description_attachments),
which are the structured metadata for files embedded in a specific rich-text
attribute. The generic attachments key is a different projection concern:
the recording's aggregate downloadable files, independent of any single
rich-text attribute. See spec/api-gaps/rich-text-attachments-coverage.md
(out-of-scope section) where this was explicitly carved out.

Scope

• Unmodeled aggregate projection.
• Low demand — file now so it isn't lost; model when demand appears.
• Only observed on the SearchResult projection (searches/show), not on the
  standalone recording GETs.

Suggested shape (when modeled)

An array member on SearchResult reusing the existing downloadable-file
metadata shape. Confirm the element schema against the jbuilder chain at
modeling time.