Importing an animated GIF as a movie (bfbdadd) made it preview and export
again, but nothing pinned it: the item must probe as a GIF movie and its
rendered frames must follow the file's own per-frame delays, not hold one
still. Add a host integration test that imports a red/green/blue GIF and
asserts each third of its duration renders the matching colour, so a
regression to still-frame import is caught.