test: add dedupe golden output contract

[CoderDeltaLAN]

Summary

• adds a representative golden output contract for the dedupe command
• covers duplicate-line console output, exit code, stdout, and stderr
• test-only change; no product behavior changed

Checks

• python -m unittest tests.test_golden_outputs tests.test_dedupe tests.test_cli
• ./scripts/check.sh
• ./scripts/post-release-audit.sh

Boundaries

No code, workflow, release, tag, PyPI, dependency, or branch-protection changes.