Stage 4 pilot (descriptive, retrospective, go/no-go only) — Fourier/modular-arithmetic transfer task, GPU engineering pilot preceding the Gate C confirmatory study

Retrospective, descriptive engineering pilot registration (not confirmatory) — frozen before any pilot result was interpreted, commit 140b6685e216b0cc34dbb20e40ab637bed1e903d. Records design and development-selection facts, not pilot outcomes; computes no p-values; never pooled with the fresh confirmatory units in Gate C. Stage 4 is GPU-only and fail-closed. Full design and provenance in the Summary field and in the linked Zenodo record.