refactor: Rename package from SwiftTUI to TUIKit

BREAKING CHANGE: Package name changed due to name collision with
existing rensbreur/SwiftTUI package.

Changes:
- Rename package from SwiftTUI to TUIKit in Package.swift
- Rename Sources/SwiftTUI to Sources/TUIKit
- Rename Sources/SwiftTUIExample to Sources/TUIKitExample
- Rename Tests/SwiftTUITests to Tests/TUIKitTests
- Rename SwiftTUI.swift to TUIKit.swift
- Update all imports: import SwiftTUI -> import TUIKit
- Update all code references: SwiftTUI.renderToBuffer -> TUIKit.renderToBuffer
- Update documentation comments
- Rename swiftTUIVersion to tuiKitVersion

All 181 tests passing.

Sources/TUIKitExample/Components/DemoSection.swift

@@ -0,0 +1,40 @@
+//
+//  DemoSection.swift
+//  TUIKitExample
+//
+//  A reusable section component for organizing demo content.
+//
+
+import TUIKit
+
+/// A section with a styled title and content.
+///
+/// Used to group related demo content with a yellow underlined title.
+///
+/// # Example
+///
+/// ```swift
+/// DemoSection("Basic Features") {
+///     Text("Feature 1")
+///     Text("Feature 2")
+/// }
+/// ```
+struct DemoSection<Content: View>: View {
+    let title: String
+    let content: Content
+
+    init(_ title: String, @ViewBuilder content: () -> Content) {
+        self.title = title
+        self.content = content()
+    }
+
+    var body: some View {
+        VStack(alignment: .leading) {
+            Text(title)
+                .bold()
+                .underline()
+                .foregroundColor(.yellow)
+            content
+        }
+    }
+}

Sources/TUIKitExample/Components/HeaderView.swift

@@ -0,0 +1,50 @@
+//
+//  HeaderView.swift
+//  TUIKitExample
+//
+//  A reusable header component for demo pages.
+//
+
+import TUIKit
+
+/// A styled header with title on the left and version on the right.
+///
+/// Used at the top of each demo page to provide consistent branding
+/// and optional subtitle.
+///
+/// # Example
+///
+/// ```swift
+/// HeaderView(
+///     title: "My Demo",
+///     subtitle: "An optional description"
+/// )
+/// ```
+struct HeaderView: View {
+    let title: String
+    let subtitle: String?
+
+    init(title: String, subtitle: String? = nil) {
+        self.title = title
+        self.subtitle = subtitle
+    }
+
+    var body: some View {
+        VStack {
+            HStack {
+                Text(title)
+                    .bold()
+                    .foregroundColor(.cyan)
+                Spacer()
+                Text("TUIKit v\(tuiKitVersion)")
+                    .dim()
+            }
+            if let sub = subtitle {
+                Text(sub)
+                    .dim()
+                    .italic()
+            }
+            Divider(character: "═")
+        }
+    }
+}