Gate C (primary confirmatory, retrospective) — Fourier/modular-arithmetic transfer: fixed-capacity counterfactual and capacity-by-complexity interaction, fresh-seed sparse confirmation

Retrospective registration; this is the PRIMARY confirmatory registration for the paper's strongest claim. Frozen before any confirmatory run was generated or any confirmatory endpoint was viewed. Implementation baseline commit c24a6bb8825fec34be5cd584b88c0e731361; pre-execution freeze commit f2ce4abf2645d52f7a134fb8887298e92bbc4612. Actual outcome reported honestly: all 4 registered fixed-capacity summaries and all 20 localization rows favored lower Fourier bandwidth, but only 1 of 4 registered capacity-by-complexity interaction tests passed -- reported as inconclusive, not evidence of no interaction. Full design in the Summary field and in the linked Zenodo record.