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>

.gitignore

@@ -6,15 +6,24 @@ xcuserdata/
 .artifacts/
 .deriveddata/
 .swift-module-cache/
+ReadViewDemo/build/
 
 # This repository checks in the Demo Pods project. Keep generated files for the
-# local RDEpubReaderView pod visible even when a global gitignore excludes Pods.
+# local RDEpubReaderView/RDPDFReaderView pods and vendored SnapKit visible even
+# when a global gitignore excludes Pods.
 !ReadViewDemo/Pods/
 !ReadViewDemo/Pods/Local Podspecs/
 !ReadViewDemo/Pods/Local Podspecs/RDEpubReaderView.podspec.json
+!ReadViewDemo/Pods/Local Podspecs/RDPDFReaderView.podspec.json
 !ReadViewDemo/Pods/Target Support Files/
 !ReadViewDemo/Pods/Target Support Files/RDEpubReaderView/
 !ReadViewDemo/Pods/Target Support Files/RDEpubReaderView/**
+!ReadViewDemo/Pods/Target Support Files/RDPDFReaderView/
+!ReadViewDemo/Pods/Target Support Files/RDPDFReaderView/**
+!ReadViewDemo/Pods/SnapKit/
+!ReadViewDemo/Pods/SnapKit/**
+!ReadViewDemo/Pods/Target Support Files/SnapKit/
+!ReadViewDemo/Pods/Target Support Files/SnapKit/**
 
 # macOS system files
 .DS_Store