3.1+ path. Structured references to product format_options[]
entries. Product-local options use { scope: 'product', format_option_id }; publisher-catalog-backed options use
{ scope: 'publisher', publisher_domain, format_option_id }.
Optionalformat_Legacy named-format path. Omitted entirely when no chosen declaration
has a v1 form; emitting [] would violate the wire schema's
minItems: 1 constraint.
Result shape for packageRefsForFormatOptions - spread into a
PackageRequestto author a 3.1+ format-option package while keeping legacy sellers working via dual emission.