fix: unmatched tracks not marked as Lost when dets_second is empty

[luigivicidomini]

Description

In ByteTrack::update, the entire second association stage was wrapped in a single if (!detections_second.empty() && ...) block. When there are no low-confidence detections around (which is pretty common), the whole block gets skipped — including the part responsible for marking unmatched tracks as Lost. Those tracks would just sit there with Tracked state and never go away, effectively becoming ghost tracks.

u_track2 is now initialized with all indices of r_tracked_stracks_ptrs before the if block, and the mark_lost loop has been moved outside of it so it always runs. Also removed u_detection2, which was being computed but never used anywhere.

Type of Change

• Bug fix (non-breaking change fixing an issue)
• New feature (non-breaking change adding functionality)
• Breaking change (fix or feature causing existing functionality to change)
• Documentation update
• Performance improvement

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have added tests that prove my fix/feature works
• New and existing unit tests pass locally
• I have updated the documentation accordingly
• My changes generate no new warnings

Testing

Tested manually by running the tracker on a video sequence where a person leaves the frame. Before the fix, the track would persist indefinitely in active_tracks_ after the person disappeared. After the fix, the track is correctly marked as Lost and eventually removed.

Screenshots (if applicable)