Add components, component sets, and instances
- SceneNode: componentId and overrides fields for instance tracking
- SceneGraph: cloneTree, createInstance, detachInstance, getMainComponent,
getInstances methods
- Editor store: createComponentFromSelection (converts frame/group or wraps
selection), createComponentSetFromComponents (groups multiple components),
createInstanceFromComponent, detachInstance, goToMainComponent
- Renderer: purple (#9747ff) selection outlines, hover highlights, size pills,
and name labels for component types; dashed purple border for component sets
- Layers panel: diamond icons for components/instances with purple tint
- Properties panel: component type indicator, Go to Main Component and
Detach Instance buttons for instances, Create Instance for components
- Keyboard shortcuts: Ctrl+Alt+K (create component), Ctrl+Shift+K
(create component set), Ctrl+Alt+B (detach instance)
- Clipboard: preserve COMPONENT/COMPONENT_SET/INSTANCE types
- Fig export: emit proper Kiwi node types instead of FRAME
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