MorphSharedElement class

Shared elements inside a flight: a piece of content (an album cover, an avatar, an icon) that TRAVELS from its place in the source to its place in the target instead of riding the fade-through crossfade.

Mark the element on both sides with the same id:

// in the button:
MorphSharedElement(id: 'cover', child: AlbumArt(size: 48))
// in the dialog content:
MorphSharedElement(id: 'cover', child: AlbumArt(size: 240))

Unlike route-level hero frameworks there is no teleportation and no second animation: both contents already live inside the one shuttle, so the flying frame is a pure function of the same spring value (geometry linear in value - interruptions stay continuous by construction), and the element is clipped by the morphing container shape like everything else.

Rules and graceful degradation:

• an id present on only one side simply renders in place;
• with snapshotGhost the source side has no live markers, so pairs do not form and the flight falls back to fade-through;
• the child must not carry a GlobalKey (it is replicated into the flying layer).