v0.3.0

Scene code without Rust plumbing (ROADMAP task 4): no Box::pin, no
BoxFuture in a typical scene.

Added

• stagger(delay, items, |x| animation): the same animation on many items,
  each starting delay seconds after the previous one.
• all(..) takes a Vec, an array or an iterator of animations of one kind
  as well as a tuple: all(cards.iter().map(|c| c.fade_in())).
• any(..): completes when the first animation does (returns its index);
  the others stop where they are.
• repeat(n, || animation) and loop_while(|| condition, || animation).
• Node helpers, timed by the theme's motion tokens: pop_in() (scale from
  zero with overshoot), slide_in(Side::Left) (slides in and fades in),
  draw_on() (outline drawn from start to end), next to the existing
  fade_in() / fade_out().
• Tween::duration(s) changes a helper's length: title.fade_in().duration(0.3).
• Clearer compile errors: all(5), all(a, b) and a stagger closure that
  ends in ; explain how to fix them.

Breaking

• sequence(delay, futures) takes any Vec, array or iterator of
  animations instead of Vec<BoxFuture>. Code that built the list with
  .collect() no longer infers its type; use stagger instead:

  // before
  sequence(0.15, cards.iter()
      .map(|c| -> oxframe::BoxFuture { Box::pin(c.opacity().to(1.0, 0.5)) })
      .collect()).await;
  // after
  stagger(0.15, &cards, |c| c.opacity().to(1.0, 0.5)).await;

  Several steps per item: return an async move { … } block from the
  closure, no boxing.

• all_vec(v) is deprecated: all(v) takes the Vec directly.

• CodeExplainer::next() returns impl Future instead of BoxFuture
  (awaiting it is unchanged).