Xcode 26.3: Native UI Shell vertical-bar toolbars do not compile #267

Xcode 26.3 (SwiftUI in the iOS 26.2 simulator SDK) does not compile ShellVerticalBars.swift.

Fix: #268

Cause

That SDK adds:

func toolbar<Content>(@ViewBuilder content: () -> Content) -> some View
func toolbar<Content>(@ToolbarContentBuilder content: () -> Content) -> some View

toolbar { } with if let + ToolbarItem + ForEach is inferred as ViewBuilder, then ToolbarItem does not conform to View. Forcing ToolbarContentBuilder still fails: ForEach is not CustomizableToolbarContent once a ToolbarItem is a sibling.

Independent of Native UI Shell swap handoff (#266). It blocks a local demo/device build of main.

Change

• nativeShellToolbar helper that calls toolbar(content:) as ToolbarContentBuilder
• ForEach of buttons inside ToolbarItemGroup (that closure is ViewBuilder)
• if let back inside ToolbarItem's view, not as a sibling of ForEach

One file, ~28 / ~28. ToolbarSpacer between groups and .axisBehavior(.verticalPreferred) on those items are dropped with that shape; they can come back if a 26.3-safe form exists.