安全区域改为从 key window 动态获取

- 新增 RDEPUBSafeArea：参考 GKNavigationBarSwift 的 keyWindow 三级查找与临时 window 兜底
- 移除 reflowableContentInsets 中 top/bottom 40 的固定默认值，改按设备真实安全区生成
- 各取安全区处的 ?? .zero 兜底改为回退到 key window 值
- DEBUG 构建下为文本可读区域绘制红色边框，方便查看排版范围

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Sources/RDReaderView/EPUBCore/RDEPUBNavigatorLayoutContext.swift

@@ -18,7 +18,7 @@ public struct RDEPUBNavigatorLayoutContext: Equatable {
         pagesPerScreen: Int = 1,
         safeAreaInsets: UIEdgeInsets = .zero,
         userInterfaceIdiom: UIUserInterfaceIdiom = .phone,
-        reflowableContentInsets: UIEdgeInsets = UIEdgeInsets(top: 40, left: 16, bottom: 40, right: 16)
+        reflowableContentInsets: UIEdgeInsets = RDEPUBSafeArea.defaultReflowableContentInsets()
     ) {
         self.containerSize = containerSize
         self.pagesPerScreen = max(1, pagesPerScreen)

Sources/RDReaderView/EPUBCore/RDEPUBSafeArea.swift

@@ -0,0 +1,67 @@
+import UIKit
+
+/// Device-level safe area lookup modeled after GKNavigationBarSwift.
+/// Reads the real safe area from the key window so correct values are
+/// available even before a view has been laid out in the hierarchy.
+/// Must be called on the main thread.
+public enum RDEPUBSafeArea {
+
+    /// Minimum text margins applied when the device safe area on an edge is
+    /// smaller (e.g. no notch / no home indicator). These are aesthetic
+    /// paddings, not approximations of the safe area itself.
+    public static let minimumVerticalTextMargin: CGFloat = 20
+
+    public static let minimumHorizontalTextMargin: CGFloat = 16
+
+    public static func keyWindow() -> UIWindow? {
+        let scenes = UIApplication.shared.connectedScenes
+            .compactMap { $0 as? UIWindowScene }
+        if let window = scenes
+            .filter({ $0.activationState == .foregroundActive })
+            .flatMap({ $0.windows })
+            .first(where: { $0.isKeyWindow }) {
+            return window
+        }
+        if let window = scenes
+            .flatMap({ $0.windows })
+            .first(where: { $0.isKeyWindow }) {
+            return window
+        }
+        return UIApplication.shared.delegate?.window ?? nil
+    }
+
+    public static func insets() -> UIEdgeInsets {
+        if let window = keyWindow() {
+            return window.safeAreaInsets
+        }
+        // No key window yet (early launch): create a detached window to read
+        // the device safe area, same fallback as GKNavigationBarSwift.
+        let window = UIWindow(frame: UIScreen.main.bounds)
+        if window.safeAreaInsets.bottom <= 0 {
+            window.rootViewController = UIViewController()
+        }
+        return window.safeAreaInsets
+    }
+
+    /// Prefers insets measured from a view already installed in the hierarchy;
+    /// falls back to the key-window insets when the view is not laid out yet
+    /// and reports .zero.
+    public static func resolve(_ measuredInsets: UIEdgeInsets?) -> UIEdgeInsets {
+        if let measuredInsets, measuredInsets != .zero {
+            return measuredInsets
+        }
+        return insets()
+    }
+
+    /// Default reflowable content insets derived from the live device safe
+    /// area instead of hard-coded heights.
+    public static func defaultReflowableContentInsets() -> UIEdgeInsets {
+        let safe = insets()
+        return UIEdgeInsets(
+            top: max(safe.top, minimumVerticalTextMargin),
+            left: max(safe.left, minimumHorizontalTextMargin),
+            bottom: max(safe.bottom, minimumVerticalTextMargin),
+            right: max(safe.right, minimumHorizontalTextMargin)
+        )
+    }
+}