feat: build Venn plots directly from DEG results

[TJoshMeyer]

Changes

• Pass the input multiOmicDataSet directly to the Venn function instead of generating an intermediate Volcano Summary table.
• Add significance and fold-change column/threshold controls matching the package API.
• Keep contrast selection optional: blank uses all DEG contrasts, while a comma-separated list selects a trimmed, duplicate-free subset.
• Remove the obsolete contrasts-column control and synchronize the vendored MOSuite Venn implementation.
• Add contract, parser, panel, success-path, subset, and unknown-contrast tests.

Issues

Closes #11. This PR is coordinated with the companion MOSuite package PR CCBR/MOSuite#278.

Generative AI Usage Statement

GitHub Copilot in Auto mode assisted with implementation, tests, troubleshooting, commit messages, and PR preparation. The changes and diffs were manually reviewed and approved by the contributor.

PR Checklist

• This comment contains a description of changes with justifications, with any relevant issues linked.
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