Shared composable state for extended components

Wasted work doing the same thing twice:

• VTreeview calls useListItems then it passes props.items to VList which calls useListItems again
• VSelect has its own selection logic but uses VList which does mostly the same thing in nested

Accessing state from child components in render, these all rely on the child being rendered before they can get values from the ref which means multiple re-renders:

• VTreeview uses children and getPath from nested through VList
• VTreeviewItem uses state from useLink and useNestedItem through VListItem
• VAppbar uses state from its VToolbar

Start by tracing the named components and composables: VTreeview, VList, VSelect, VTreeviewItem, VAppbar, useListItems, useLink, and useNestedItem. Determine how shared state could avoid duplicate work and render-order-dependent re-renders; done means the affected components share the relevant state without repeated composable work.