Visible clip thumbnails wait behind offscreen decodes #2706

Opening the GPUI Clips sidebar queues thumbnail decodes for every timeline clip, in order, with two jobs in flight. Scrolling down while those jobs run leaves newly visible cards waiting behind offscreen work. The cache eventually retains thumbnails for the entire list even if the user never visits those cards.

A native replay with a synthetic 1080p H.264 recording confirms the behavior. With 100 clips and a 390 × 600 viewport, an idle sidebar decodes 100 thumbnails; requesting only visible cards decodes 8. After visiting the top and bottom of a 500-clip list, the current code retains 500 thumbnail images instead of the 16 actually visited.

Request visible thumbnails using the existing card clipping bounds while preserving the decoder, cache keys, failure handling and two-job concurrency limit.

I have a verified patch ready to push. Checked open PRs and issues for overlapping work; #2705 addresses list rendering rather than thumbnail requests.