Support multiple output products from fold nodes #947

Summary

Allow fold nodes to produce multiple output data products.

Current behavior

Fold registration and implementation currently assume exactly one output product. The fold API declares num_outputs = 1, and the fold node builds its output specification from a single result type.

Desired behavior

A fold algorithm should be able to return multiple output objects from its final result, with each output product assigned a corresponding suffix through output_product_suffixes(...).

Acceptance criteria

• Fold algorithms can define multiple output product types.
• Fold output specifications map each returned object to its corresponding output suffix.
• Registration validates that the number of specified suffixes matches the number of returned products.
• Tests verify that downstream algorithms can consume each output product.