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/TUIKit/Modifiers/PaddingModifier.swift

@@ -0,0 +1,164 @@
+//
+//  PaddingModifier.swift
+//  TUIKit
+//
+//  The .padding() modifier for adding space around a view.
+//
+
+/// Edge insets defining padding on each side.
+public struct EdgeInsets: Sendable, Equatable {
+    /// Padding above the content.
+    public var top: Int
+
+    /// Padding to the left of the content.
+    public var leading: Int
+
+    /// Padding below the content.
+    public var bottom: Int
+
+    /// Padding to the right of the content.
+    public var trailing: Int
+
+    /// Creates edge insets with individual values.
+    public init(top: Int = 0, leading: Int = 0, bottom: Int = 0, trailing: Int = 0) {
+        self.top = top
+        self.leading = leading
+        self.bottom = bottom
+        self.trailing = trailing
+    }
+
+    /// Creates uniform edge insets.
+    ///
+    /// - Parameter value: The padding on all four sides.
+    public init(all value: Int) {
+        self.top = value
+        self.leading = value
+        self.bottom = value
+        self.trailing = value
+    }
+
+    /// Creates horizontal and vertical edge insets.
+    ///
+    /// - Parameters:
+    ///   - horizontal: The padding on leading and trailing sides.
+    ///   - vertical: The padding on top and bottom sides.
+    public init(horizontal: Int = 0, vertical: Int = 0) {
+        self.top = vertical
+        self.leading = horizontal
+        self.bottom = vertical
+        self.trailing = horizontal
+    }
+}
+
+/// The edges of a view.
+public struct Edge: OptionSet, Sendable {
+    public let rawValue: UInt8
+
+    public init(rawValue: UInt8) {
+        self.rawValue = rawValue
+    }
+
+    /// The top edge.
+    public static let top = Edge(rawValue: 1 << 0)
+
+    /// The leading (left) edge.
+    public static let leading = Edge(rawValue: 1 << 1)
+
+    /// The bottom edge.
+    public static let bottom = Edge(rawValue: 1 << 2)
+
+    /// The trailing (right) edge.
+    public static let trailing = Edge(rawValue: 1 << 3)
+
+    /// All edges.
+    public static let all: Edge = [.top, .leading, .bottom, .trailing]
+
+    /// Horizontal edges (leading and trailing).
+    public static let horizontal: Edge = [.leading, .trailing]
+
+    /// Vertical edges (top and bottom).
+    public static let vertical: Edge = [.top, .bottom]
+}
+
+/// A modifier that adds padding around a view.
+public struct PaddingModifier: ViewModifier {
+    /// The padding insets.
+    public let insets: EdgeInsets
+
+    public func modify(buffer: FrameBuffer, context: RenderContext) -> FrameBuffer {
+        var result: [String] = []
+
+        let leadingPad = String(repeating: " ", count: insets.leading)
+        let trailingPad = String(repeating: " ", count: insets.trailing)
+
+        // Top padding
+        let lineWidth = buffer.width + insets.leading + insets.trailing
+        let emptyLine = String(repeating: " ", count: lineWidth)
+        for _ in 0..<insets.top {
+            result.append(emptyLine)
+        }
+
+        // Content lines with horizontal padding
+        for line in buffer.lines {
+            result.append(leadingPad + line + trailingPad)
+        }
+
+        // Bottom padding
+        for _ in 0..<insets.bottom {
+            result.append(emptyLine)
+        }
+
+        return FrameBuffer(lines: result)
+    }
+}
+
+// MARK: - View Extension
+
+extension View {
+    /// Adds padding on all sides.
+    ///
+    /// ```swift
+    /// Text("Hello")
+    ///     .padding(2)
+    /// ```
+    ///
+    /// - Parameter amount: The padding amount on all sides (default: 1).
+    /// - Returns: A padded view.
+    public func padding(_ amount: Int = 1) -> ModifiedView<Self, PaddingModifier> {
+        modifier(PaddingModifier(insets: EdgeInsets(all: amount)))
+    }
+
+    /// Adds padding on specific edges.
+    ///
+    /// ```swift
+    /// Text("Hello")
+    ///     .padding(.horizontal, 4)
+    /// ```
+    ///
+    /// - Parameters:
+    ///   - edges: The edges to pad.
+    ///   - amount: The padding amount (default: 1).
+    /// - Returns: A padded view.
+    public func padding(_ edges: Edge, _ amount: Int = 1) -> ModifiedView<Self, PaddingModifier> {
+        let insets = EdgeInsets(
+            top: edges.contains(.top) ? amount : 0,
+            leading: edges.contains(.leading) ? amount : 0,
+            bottom: edges.contains(.bottom) ? amount : 0,
+            trailing: edges.contains(.trailing) ? amount : 0
+        )
+        return modifier(PaddingModifier(insets: insets))
+    }
+
+    /// Adds padding with explicit edge insets.
+    ///
+    /// ```swift
+    /// Text("Hello")
+    ///     .padding(EdgeInsets(top: 1, leading: 4, bottom: 1, trailing: 4))
+    /// ```
+    ///
+    /// - Parameter insets: The edge insets.
+    /// - Returns: A padded view.
+    public func padding(_ insets: EdgeInsets) -> ModifiedView<Self, PaddingModifier> {
+        modifier(PaddingModifier(insets: insets))
+    }
+}