Fix add_clips leaving stranded audio when overwriting linked clips

[PriyeshPandey2000]

clearRegion calls splitClip (which splits linked partners on both tracks) then removeClips (which only removes the given clip id). When add_clips overwrites a linked V+A pair, the audio fragment in the cleared range is never removed. placeClip then finds the audio track occupied and creates a spurious second audio track, causing the original audio to play under the new clip.

Fix collects linked partner track IDs before clearing the video track, then issues a second clearRegion on each partner track. placeClip finds the audio track clear and routes the new audio there correctly.

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Note

Medium Risk
Targeted agent timeline overwrite path for linked V+A; wrong partner detection could over-clear audio, but scope is narrow and covered by a new test.

Overview
Fixes add_clips when it overwrites part of an existing linked video+audio pair on the timeline.

Before placing each clip, the handler still clearRegion on the target video track, but it now pre-collects linked partner tracks for clips overlapping that range and runs clearRegion on those tracks too. That removes audio fragments that used to survive after a video-only clear (split partners, remove only the video piece), which led to double playback and placeClip spawning an extra audio track.

Adds regression test addClipsClearsLinkedAudioWhenOverwritingLinkedPair asserting two tracks stay, three audio segments, and the middle segment uses the new asset.

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