Drop decoder assumptions in the generic non‑universality argument

Develop an analog of the periodic‑point argument proving generic non‑universality that removes all conditions on the decoder, and in particular establish the result even when the decoder is merely shrinking (rather than hierarchically shrinking).

Background

The authors prove Theorem 4.3: an open dense set of diffeomorphisms cannot be extended to robustly Turing‑universal CDSs under the combined assumptions of a hierarchically shrinking decoder and constant slowdown. They remark that their periodic‑point argument relies crucially on these assumptions.

They explicitly state a gap: extending the argument to settings with weaker decoder assumptions (shrinking or no condition) remains unresolved, highlighting a technical barrier in removing encoder/decoder structure from the dynamical non‑universality proof.