Add abstract interface for multiple extended-object trackers

[FlorianPfaff]

Summary

• Adds AbstractMultipleExtendedObjectTracker, inheriting from AbstractMultitargetTracker.
• Adds structured MEOT dataclasses: ExtendedObjectEstimate, ExtendedObjectAssociationResult, and MultipleExtendedObjectStepResult.
• Supports vectorized point-estimate compatibility, structured object extraction, contour extraction, measurement-rate histories, cardinality histories, and extended-object association histories.
• Exports the new interface from pyrecest.filters.

Rationale

Multiple extended-object tracking differs from point-target MTT because one object can generate multiple detections in one scan. The association interface therefore models object-to-measurement-cell assignments, clutter cells, birth cells, and optional global hypotheses rather than only one-to-one track/measurement pairs.

Tests

• Adds focused unit tests for vectorized estimate output, structured object estimates, measurement-rate extraction, contour labels, step results, and history logging.

Notes

This PR adds the abstract interface only; concrete GGIW/PMBM/GLMB/track-manager implementations can build on it in follow-up changes.

[github-actions]

✅MegaLinter analysis: Success

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