The endpoint inventory treats 404 as "absent", but the spec explicitly assigns 404 to both missing resources and unsupported endpoints (see docs/spec-gaps.md).
Harden classification: on a 404, re-probe with a well-formed input and only classify as not-implemented if the second probe also indicates absence. Reference and tranquil-pds currently disambiguate cleanly via 501/400/401, so this is robustness rather than a live bug.
Done when: the inventory uses two-stage probing and the classification logic is covered by a comment pointing at the spec ambiguity.