v0.6.0: Blend one scene into the next.

Scenes can blend into each other (ROADMAP task 5).

Added

• transition(Transition::slide(Dir::Left)).await at the start of a scene.
  Also fade, wipe, zoom_in, zoom_out, .duration, .ease, .overlap,
  and Transition::custom. The length is stored in meta.json and can be
  dragged on the timeline. render --scene leaves the incoming blend out.
  check samples it and inspect reports kind and progress.
• Example transitions: a title card slides into an end card.
• The browser presents a transition as one canvas frame. Both layers are drawn first, then blended.