v1.47.0

Build: pin the bundled ffmpeg, and drop -vsync from smoke (v1.47.0)

The first v1.47.0 build failed on both platforms: requirements.txt said
imageio-ffmpeg>=0.5, so CI installed a newer wheel carrying ffmpeg 8,
which removed -vsync, and six of smoke's frame-extraction checks died
on it. Nothing shipped -- no release was created.

The real problem was the unpinned wheel: it is the ffmpeg every user's
app bundles, and it had silently jumped a major version past everything
golden and the byte-identical timer check were run on (7.1). It is now
pinned to the tested 0.6.0. Smoke's extraction uses -fps_mode
passthrough instead, which works on 7.1 and 8 alike. Smoke passes
locally (697).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>