Concept
Four findings, and a failure is never absence
The resolver returns exactly ok / absent / unreachable / invalid; a transport failure resolves to unreachable and never collapses to absent, because “could not reach” is not “is not there”.
Pinned to
agent-dns/1 — the discovery naming layerThe canonical source this concept is taught from. If it moves, the lesson is regenerated — teaching is pinned to code, not retyped.
On the frontier
No course teaches this concept yet — it is a named gap on the content graph, not a hole nobody sees. A gap is the roadmap, not an embarrassment: the count only falls by teaching.