修复 PDF 仿真翻页模式下翻页后页面全白

打开普通 PDF 并使用仿真翻页阅读时，除首页外每翻一页都是纯白：没有页面
内容，也没有页面四周的衬底色。横向滚动与竖滑模式不受影响。样张
英汉大词典App需求说明.pdf 上第 2 到第 9 页无一例外。

根因是页面数据回来时找不到对应的视图。UIPageViewController 会通过
viewControllerBefore/After 提前构建相邻页的内容视图，构建时页面描述尚未
就绪，configure 发起异步 readerPage 请求后先返回一张空白页。描述返回后
acceptPageDescriptor 调用 refreshVisiblePage，而它经由
RDPDFReaderView.pageContentView(pageNum:) 定位视图，该方法在仿真翻页下
只检索 nativeCurlPageController.viewControllers，也就是已经上屏的 child。
预建的 child 此时还不在其中，图片因此从未被装配上去；翻页动画结束后又
没有任何路径重新 configure 该视图，白页就固定下来。此前偶尔能看到内容，
是 OCR 完成回调顺带触发了一次刷新，并非稳定行为。

改为按页码刷新该页的全部活动视图：控制器用弱引用表记录已创建的页面视图，
refreshVisiblePage 改名为 refreshPageViews(at:)，除已上屏视图外一并装配
尚未上屏的预建视图，pageContentView 创建时即写入 tag 以便按页码匹配。
翻页落定时在 pageNum 回调中补一次装配，使内存告警清掉页面描述的场景也能
重新发起请求并恢复。另外把 image 为空的页面描述改按无效结果处理，走既有
的重试与失败重载入口，不再缓存出一张永远不会重试的白页。

新增 PDFPageTurnBlankPageTests 作为回归：翻页稳定后截屏统计画面中部深色
像素占比，白页为 0，扉页约 0.002，正文页在 0.01 以上，以 0.0005 为判定
阈值，覆盖仿真翻页正翻、仿真回翻和横向滚动三种路径。

验证：iPhone 17 Pro / iOS 26.4.1 模拟器执行 PDFPageTurnBlankPageTests、
PDFAnnotationTests、PDFDrawingModeTests、PageNavigationTests、
DisplayTypeTests 共 23 条用例，22 条通过。修复前仿真翻页第 2 到 9 页墨迹
占比全为 0.0000，修复后与横向滚动模式逐页吻合。唯一失败的
DisplayTypeTests.testDisplayTypeSwitchMidSession 属 EPUB 设置面板既有问题，
将本次改动还原后同样失败。文档补充了 Demo 因链接 FoundationModels 而必须
使用 iOS 26 及以上模拟器的说明。

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

ReadViewDemo/ReadViewDemo.xcodeproj/project.pbxproj

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@@ -213,6 +215,7 @@
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 				1A2B3C4D00000008AABBCC01 /* PageNavigationTests.swift */,
 				A1C5E34F6B7192D400A1B235 /* PDFAnnotationTests.swift */,
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