* test(engine): the URL-read audio mix test stops its mix before cleanup and is not timed
* test(studio): bank the open-film thumbnail counts on the CI Chrome
The CI Chrome moved to 155 and renders one thumbnail fewer: 13 renders, 14 requests.
* fix(engine): a long audio mix prepares its clips a few at a time
A 200-clip mix started 200 ffmpeg at once and starved the host.
* chore(engine): trim the long comments in the audio mixer
Reasoning moved to the PR.
* test(engine): the mix order test starts every preparation at once
The mixer prepares clips in a core-sized pool; the test now sets enough cores.