rebuild_timeline synchronously extracted every audio clip's waveform,
and the extractor decodes the clip's ENTIRE audio inline — with two
40-minute files on the timeline, opening the project froze the UI for
seconds ("the project takes a while to open"). Extraction now runs on
a background thread; the cache bumps a version counter on insert and
the engine tick repaints the timeline when it changes.
Mike-Solar
96df9293e5