MorphSkin class

A group of pieces with one shared "skin": the smooth union of their SDFs traced into a single Path behind live content. Nearby pieces fuse with a concave fillet on their own; the blend width comes from style or an explicit blend (low - a geometric joint, high - a gooey neck).

"One mass - one shadow": elevation is drawn as a single shadow of the unified contour, so pieces cannot visually split into layers.

The flight neck comes for free: the group finds flights launched in the MorphScope under its piece ids (including a declarative MorphAnchor with an explicit tagId) and plays the airborne blob under the shuttle, bridge detachment, and the landing squash of the skin by itself. Outside a MorphScope the group degrades to pure fusion.

Implemented as a render object: spring ticks mark paint only - no widget rebuild, no relayout participates in an animation frame. App-driven geometry gets the same citizenship through MorphPieceChannel: a channel write repaints and re-traces without touching the widget tree. The landing squash (and the channel delta) is a paint transform of the content, mirroring the skin's mass deformation. The group is its own repaint boundary, so an animating skin never repaints its ancestors.