The ParamNotInRestrictedSet diagnostic named the offending type, the
type-variable and the home type, but not the allowed set {Int, Float,
Bool} that design/models/0007 §4 promises it names. A downstream author
was told what is wrong but not what is right, and the kernel source that
carries the set is not available to a downstream consumer.
The allowed set already travels on the error struct (it flows into the
JSON `details.allowed`); only the human-readable `#[error]` render
dropped it. Append it to the Display string. JSON details unchanged.
Surfaced by the raw-buf fieldtest (finding B3, docs/specs/0058).
RED-first: param_in_reject_message_names_allowed_set pins the message
content before the one-line render fix.
Brummel
75109f171d