Featherweight timeline

Improvements:

• The timeline used to redraw all its clips, ticks, and waveforms on every single frame of playback — on caption-heavy projects that alone consumed the whole frame budget. It now updates only the playhead during playback (roughly 90× cheaper), so busy projects play smoothly.
• Sources bigger than your project — like a 4K clip in a 1080p edit — now decode at project resolution during preview, cutting memory use 4×. Exports still use the full original quality.