Add split-and-connect tracklet postprocessing

[mikel-brostrom]

Summary

• add a split-and-connect tracklet (SCT) postprocessing pipeline for MOT result folders
• expose the SCT helper through the package API, CLI, and documentation alongside existing GSI support
• cover SCT merging with a new unit test

Testing

• python -m pytest tests/unit/test_postprocessing.py (fails: numpy unavailable in the execution environment)

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Codex Task

[Copilot]

@mikel-brostrom I've opened a new pull request, #2177, to work on those changes. Once the pull request is ready, I'll request review from you.

[Copilot]

@mikel-brostrom I've opened a new pull request, #2178, to work on those changes. Once the pull request is ready, I'll request review from you.