Add LabelGroup support

Sources/OpenSwiftUI/View/LabelGroup/BodyLabelGroupStyle.swift

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+//
+//  BodyLabelGroupStyle.swift
+//  OpenSwiftUI
+//
+//  Audited for 6.5.4
+//  Status: Complete
+
+// MARK: BodyLabelGroupStyle
+
+struct BodyLabelGroupStyle: LabelGroupStyle_v0 {
+    func font(at level: Int) -> Font {
+        switch level {
+        case 0: .body
+        case 1: .subheadline
+        case 2: .footnote
+        default: .footnote
+        }
+    }
+
+    func foregroundStyle(at level: Int) -> HierarchicalShapeStyle {
+        switch level {
+        case 0: .primary
+        case 1, 2: .secondary
+        default: .tertiary
+        }
+    }
+}

Sources/OpenSwiftUI/View/LabelGroup/LabelGroup.swift

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+//
+//  LabelGroup.swift
+//  OpenSwiftUI
+//
+//  Audited for 6.5.4
+//  Status: Complete
+
+@_spi(Private)
+@available(OpenSwiftUI_v4_0, *)
+public struct LabelGroup<Content>: View where Content: View {
+    var content: Content
+
+    public init(@ViewBuilder content: () -> Content) {
+        self.content = content()
+    }
+
+    public var body: some View {
+        ResolvedLabelGroupStyle()
+            .viewAlias(LabelGroupStyleConfiguration.Content.self) {
+                content
+            }
+    }
+}
+
+@_spi(Private)
+@available(*, unavailable)
+extension LabelGroup: Sendable {}

Sources/OpenSwiftUI/View/LabelGroup/LabelGroupStyle.swift

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+//
+//  LabelGroupStyle.swift
+//  OpenSwiftUI
+//
+//  Audited for 6.5.4
+//  Status: WIP
+//  ID: D603B70BAAC92B7EC91530E54167B115 (SwiftUI)
+
+// MARK: - LabelGroupStyle_v0
+
+@_spi(UIFrameworks)
+@available(OpenSwiftUI_v6_0, *)
+public protocol LabelGroupStyle_v0 {
+    associatedtype Foreground: ShapeStyle = HierarchicalShapeStyle
+
+    func font(at level: Int) -> Font
+
+    func foregroundStyle(at level: Int) -> Self.Foreground
+}
+
+// MARK: - View + labelGroupStyle_v0
+
+@_spi(UIFrameworks)
+@available(OpenSwiftUI_v6_0, *)
+extension View {
+    nonisolated public func labelGroupStyle_v0(_ style: some LabelGroupStyle_v0) -> some View {
+        modifier(LabelGroupStyleModifier(style: style))
+    }
+}
+
+// MARK: - LabelGroupStyleConfiguration
+
+struct LabelGroupStyleConfiguration {
+    struct Content: ViewAlias {}
+
+    let content: Content
+}
+
+// MARK: - LabelGroupStyleModifier
+
+struct LabelGroupStyleModifier<S>: StyleModifier where S: LabelGroupStyle_v0 {
+    var style: S
+
+    init(style: S) {
+        self.style = style
+    }
+
+    func styleBody(configuration: LabelGroupStyleConfiguration) -> some View {
+        _VariadicView.Tree(StyleApplicator(style: style)) {
+            configuration.content
+        }
+    }
+}
+
+// MARK: - ResolvedLabelGroupStyle
+
+struct ResolvedLabelGroupStyle: StyleableView {
+    static let defaultStyleModifier: LabelGroupStyleModifier<BodyLabelGroupStyle> = .init(style: .init())
+
+    var configuration: LabelGroupStyleConfiguration { .init(content: .init()) }
+}
+
+// MARK: - StyleApplicator [WIP]
+
+private struct StyleApplicator<S>: _VariadicView.MultiViewRoot where S: LabelGroupStyle_v0 {
+
+    struct EnumeratedView {
+        var view: _VariadicView.Children.Element
+        var offset: Int
+    }
+
+    var style: S
+
+    func body(children: _VariadicView.Children) -> some View {
+        ForEach(
+            children.enumerated().map { EnumeratedView(view: $0.element, offset: $0.offset) },
+            id: \.view.id
+        ) { view in
+            view.view
+            // defaultForegroundStyle
+            // font
+            // platformItem
+        }
+    }
+}