Transitions: animate views as they appear and disappear

[colemancda]

The deferred half of the animation tier: .transition(_:) animates a view as it is inserted into or removed from a stack (typically inside withAnimation).

What's new

• AnyTransition — .opacity, .slide, .scale, .move(edge:), .identity; .transition(_:) modifier.
• The interpreter wraps a transition-carrying child of a VStack/HStack/ZStack in an AnimatedVisibility whose enter/exit is derived from the transition kind (timing follows the active withAnimation spec, else a 300ms tween).

The interesting part

A child removed from the Swift tree is simply gone from node.children, so there's nothing left to animate out. Each stack keeps a slot per transition child (its last node + a MutableTransitionState); when the child leaves, its slot lives on with targetState = false so the exit plays, then evicts when idle. Crucially, present and exiting children render through one unified key(id)-ed loop — my first cut used two loops, and the child hopping call sites made Compose tear it down instead of animating (bug caught on-device via a state-log; the two-loop version removed instantly).

Verification

• swift test — new emission test, 66 total passing
• Emulator (Animation screen, new "Transition: move + fade" and "Transition: scale" examples), frame-sampled:
  • Enter: the box slides in from the leading edge (caught at x≈0–30 mid-flight before settling)
  • Exit: a mid-frame shows the box slid left and clipped off the leading edge ("…rom the leading edge and fade"), and the MutableTransitionState log shows cur animating true→false over ~400ms — a real exit, not a snap

Deferred

Asymmetric transitions (.asymmetric), .combined(with:), and perfectly in-place exit ordering when multiple siblings leave at once (exiting children are appended, which is in-place for the common single-optional-child case).