Companion

The RFC's built-in problem type, and the sole initial entry in the "HTTP Problem Types" registry (§4.2.1): "this problem has no more specific semantics beyond the HTTP status code itself". §3.1 makes it the meaning of an absent type, so emitting it explicitly and omitting the member are equivalent to a conforming consumer.

How deeply an extension value may nest before either codec refuses it.

The five member names §3.1 defines, which no extension member may reuse.

The about:blank problem for status. title SHOULD be the status code's standard reason phrase (§4.2.1, e.g. "Not Found" for 404). This module carries no reason-phrase table, so the caller passes one in. problem-details-ktor adds an overload that fills it from HttpStatusCode.description.