新增全书字符统计与阅读百分比,并沉淀后台渲染流水线优化方案

- 新增 RDEPUBTextStatisticsCoordinator:后台单线程按章渲染统计全书字符数,footer 显示精确阅读百分比;统计未就绪时(含 PageMap 未覆盖的 loading 页)显示"计算中",不再回退旧页码
- 统计结果按 bookID + renderSignature + 章节内容哈希做磁盘缓存,布局/字号变化自动失效;缓存命中不创建 renderer
- 移除"0.8 秒翻页时间窗轮询让路"逻辑,后续由事件驱动的交互加载门替代(见方案文档 5.6 节)
- 新增 Doc/FeatureSolution/TEXT_STATISTICS_PIPELINE_OPTIMIZATION.md:三条渲染流水线收敛为"交互 + 生产者"、交互加载门、全局后台渲染预算池、后台渲染登记表、两阶段近似百分比、自适应并发的完整优化方案与验收标准

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

Sources/RDReaderView/EPUBUI/TextPage/RDEPUBTextContentView.swift

@@ -360,7 +360,8 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
         chapterCFIMap: RDEPUBCFIMap? = nil,
         chapterFragmentOffsets: [String: Int] = [:],
         highlights: [RDEPUBHighlight] = [],
-        searchState: RDEPUBSearchState? = nil
+        searchState: RDEPUBSearchState? = nil,
+        footerText: String? = nil
     ) {
         loadingSpinner.stopAnimating()
         currentPage = page
@@ -378,7 +379,7 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
         )
         backgroundColor = configuration.theme.contentBackgroundColor
         pageNumberLabel.textColor = configuration.theme.contentTextColor
-        pageNumberLabel.text = "\(pageNumber) / \(totalPages)"
+        pageNumberLabel.text = footerText ?? "\(pageNumber) / \(totalPages)"
 
         if configureCoverIfNeeded(for: page) {
 #if canImport(DTCoreText)
@@ -446,7 +447,8 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
     func configureLoading(
         pageNumber: Int,
         totalPages: Int,
-        configuration: RDEPUBReaderConfiguration
+        configuration: RDEPUBReaderConfiguration,
+        footerText: String? = nil
     ) {
         currentPage = nil
         currentChapterCFIMap = nil
@@ -463,7 +465,7 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
         )
         backgroundColor = configuration.theme.contentBackgroundColor
         pageNumberLabel.textColor = configuration.theme.contentTextColor
-        pageNumberLabel.text = "\(pageNumber) / \(totalPages)"
+        pageNumberLabel.text = footerText ?? "\(pageNumber) / \(totalPages)"
         coverImageView.isHidden = true
         coverImageView.image = nil