Opaque DXN struct value, produced when no schema is compiled for its name (DXN.md §1.4: "a reader lacking the schema... returns an opaque tagged value rather than failing").

fields is one of two shapes, and deliberately not normalized to a single common one — .dxn's keyed literal form carries field names; .dxnb's wire form is always positional (DXN.md §2.1) and never carries names at all. Without a schema there is no way to recover one shape from the other (a positional .dxnb struct has no names to reconstruct), so Dextrin.Struct says which shape it actually is rather than pretend they're interchangeable. This is also why an opaque struct decoded from .dxnb can't be cross-checked against the same struct decoded from .dxn without a schema in hand — there's nothing to compare field-by-field yet.