将 PDF 成品阅读控制器下沉为 RDPDFReaderView SDK，并修复手机横屏单页连续竖滑

SDK 化收尾：
- 新增 RDPDFReaderView 本地 pod（阅读容器、成品控制器、划线/注释、画笔、
  目录/设置面板、主题与持久化契约），Demo 精简为 PDFKit 页面提供者 + 持久化宿主
- Podfile/工程接入 RDPDFReaderView 与 SnapKit，示例书改用 PDF 阅读器示例文件

手机横屏单页竖滑修复：
- cell 高度改为宽度适配后的纸张高度（sizeForItemAt + 宿主按页提供纵横比），
  纵向超出一屏的内容由外层 collectionView 连续滚动承接，不再被裁掉
- 页面内部不再开启同向嵌套的兜底竖向拖动，修复外层滚动被吞、无法滚到下一页
- 修复 applyContentSize 先评估拖动开关后写 contentSize 的顺序缺陷
- 页码按视口中心命中 cell 计算，补 didEndDragging 兜底；旋转重锚点与
  transitionToPage 改用真实布局位置
- 横竖屏同为竖滑时也重建 cell 并刷新全部可见页的适配配置
- 横屏单页点击不再跳页，只显隐工具栏；翻页交给连续竖滑

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

ReadViewDemo/Pods/SnapKit/Sources/ConstraintLayoutSupportDSL.swift

@@ -0,0 +1,56 @@
+//
+//  SnapKit
+//
+//  Copyright (c) 2011-Present SnapKit Team - https://github.com/SnapKit
+//
+//  Permission is hereby granted, free of charge, to any person obtaining a copy
+//  of this software and associated documentation files (the "Software"), to deal
+//  in the Software without restriction, including without limitation the rights
+//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+//  copies of the Software, and to permit persons to whom the Software is
+//  furnished to do so, subject to the following conditions:
+//
+//  The above copyright notice and this permission notice shall be included in
+//  all copies or substantial portions of the Software.
+//
+//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+//  THE SOFTWARE.
+
+#if canImport(UIKit)
+    import UIKit
+#else
+    import AppKit
+#endif
+
+
+@available(iOS 8.0, *)
+public struct ConstraintLayoutSupportDSL: ConstraintDSL {
+    
+    public var target: AnyObject? {
+        return self.support
+    }
+    
+    internal let support: ConstraintLayoutSupport
+    
+    internal init(support: ConstraintLayoutSupport) {
+        self.support = support
+        
+    }
+    
+    public var top: ConstraintItem {
+        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.top)
+    }
+    
+    public var bottom: ConstraintItem {
+        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.bottom)
+    }
+    
+    public var height: ConstraintItem {
+        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.height)
+    }
+}