Import: test that an animated GIF imported from disk plays (#651)

Animated GIFs came in as one still frame in 0.7.0; bfbdadd3 (after that
release) probes them as movies. This end-to-end test imports a GIF file
through the wired session into a comp and checks the layer shows each of
its frames at its time, the path the viewer and exports share.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QiAJeFttZp83Fh7YuzAR5f

Cargo.lock

@@ -1350,6 +1350,7 @@ dependencies = [
  "effectcraft-svg",
  "effectcraft-time",
  "exr",
+ "gif",
  "image",
  "rayon",
  "serde_json",