* fix(core): audio that decodes short plays from its element in preview
* fix(core): keep the decoded copy when the element's duration is unknown
* fix(player): preview pauses with Buffering until every track due can play
* fix(core): keep the transport awake while it buffers
* fix(core): wait only for tracks the clock would also treat as playable
* test(core): buffering fixture pauses like a browser, with a pause event
* fix(core): hold for a clip only after media sync has asked it to play
* test(core): hold lands on the frame a slow tick skips into a clip with no data
* fix(core): a held clip counts as ready once it has data buffered ahead