Wire helper-mode sparse indices into DisTrO aggregation

[plugyawn]

Problem

MatFormer helper mode includes align_matformer_sparse_grad, but it’s not wired into DisTrO or the network payloads. Helper indices are not transmitted, and the aggregation path only aligns contiguous prefix grads.

Refs:

• shared/modeling/src/distro.rs (TODO added above align_matformer_sparse_grad)

Expected

If helper mode uses sparse index selection, the receiver must scatter into full shapes for aggregation.

Possible Approach

• Extend DistroResult/payloads to include helper indices (or a reproducible seed/epoch for reconstruction).
• Use align_matformer_sparse_grad in the hetero-aggregation path.
• Add tests for helper-mode sparse aggregation.

Acceptance Criteria

• Helper mode works with sparse gradient transmission.
• Aggregation produces correct full-shape grads.
• Tests validate helper-mode wiring end-to-end.