Fix tests after the community merges: GPU Pixel Motion Blur without a host, timeline footage kind, effects doc

- GPU Pixel Motion Blur (#695) declined without a host; it now averages the shutter's
  frames like the CPU effect (tests_fx_time::without_a_host).
- add_item_keeps_footage_longer_than_the_comp_at_full_length (#798) built footage with
  the default kind (Still), which adds at comp length; it is a movie.
- docs/effects.md regenerated.

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

crates/engine/src/tests_timeline.rs

@@ -973,6 +973,7 @@ fn add_item_keeps_footage_longer_than_the_comp_at_full_length() {
     let f = effectcraft_project::Footage {
         width: 100,
         height: 100,
+        kind: effectcraft_project::FootageKind::Video,
         has_video: true,
         duration: effectcraft_time::Tick::from_seconds_f64(10.0),
         ..Default::default()