The Tester as Witness. You've done inspector, scientist, user, explorer, investigative journalist. The legal metaphor is conspicuously absent. A witness does not argue the case, does not render the verdict, and is not the prosecution — but their testimony is what the court's judgment rests on. This cleanly dissolves the "tester as gatekeeper" confusion you raised in Five Essential Lessons. Hume on testimony[1] and C.A.J. Coady's Testimony: A Philosophical Study[2] are obvious references.

[1] https://plato.stanford.edu/entries/hume/#AnEnHuUn
[2] https://global.oup.com/academic/product/testimony-9780198235514