fn font_identity(font: Font) -> &'static [u8] ⓘExpand description
A backend-neutral, stable identity for a font face.
Deliberately not Font::pdf_resource_name (F1..): that is a PDF emitter
resource slot, and a layout signature must not depend on backend layout. The
returned tag is owned by this boundary and stable forever; the exhaustive
match means a new bundled face fails to compile here until it is assigned a
tag, so the identity can never silently alias.