test: assert deterministic close-period preview across shuffled holders

[pchieneye]

test: assert deterministic close-period preview across shuffled holders

Summary

This PR adds a regression test to ensure close-period preview distribution output is deterministic regardless of holder-list ordering.

What changed

• Added a regression test covering shuffled holder-order inputs for close-period preview behavior.
• Added a deterministic shuffle helper for reproducible test fixtures.
• Canonicalized the preview payout ordering so the distribution output remains stable across equivalent holder sets.

Why

The close-period preview should not depend on the incoming order of holders. This change ensures that the same set of holders produces the same preview/distribution result even when the input order is shuffled.

Validation

• Added a targeted regression test for the deterministic ordering behavior.
• Kept the change scoped to the close-period preview logic and its test coverage.

Closes: #602

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