Block MatFormer helper mode (suffix sampling) until rotation/sparse alignment is wired #7
Description
Helper-mode suffix sampling is currently hard-disabled via assert. We should only re-enable after: (1) set_round is wired to trainer step for rotation; (2) helper index seeding uses a stable hash across toolchains; (3) DisTrO can align sparse helper indices (or indices are transmitted). Add tests for rotation + bandwidth impact.
Context: helper mode currently risks fixed suffix subsets and misaligned gradients.