feat: 架构整改 — Context拆分、Runtime拆分、异步章节加载、UI测试覆盖

Phase 1: Context 拆分
- 新增 RDEPUBReaderState/RDEPUBReaderEnvironment/RDEPUBReaderServices
- RDEPUBReaderContext 改为过渡门面，代理到 State/Environment/Services

Phase 2: Runtime 拆分
- 新增 RDEPUBPresentationRuntime 处理分页状态管理
- 新增 RDEPUBChapterWarmupOrchestrator 处理章节预热与加载编排
- RDEPUBReaderRuntime 从 1277 行收缩，公共 API 转发到新 facade

Phase 0.5: 性能优化
- prepareOnDemandChapter 支持异步模式（allowSynchronousLoad: false）
- extendPartialBookPageMapIfNeeded 改为 DispatchGroup 并发加载
- RDEPUBChapterOffsetMap.cfiMap 加 NSLock 保护数据竞争
- CFI Map 构建延迟到后台队列（scheduleDeferredCFIMapBuildIfNeeded）
- RDEPUBTextPageRenderView 引入静态位图缓存
- RDEPUBTextContentView 新增 loadingSpinner 占位页

Phase 3: 状态机
- 新增 RDEPUBNavigationStateMachine（含 DEBUG 合法转换校验）
- 新增 RDEPUBPaginationState 记录分页来源

Review 修复
- makeSummary 重复方法合并
- ensureNavigationTargetAvailable 同步路径加注释标记

UI 测试
- 新增 AsyncChapterLoadingTests（20 个测试，覆盖全部架构整改场景）
- 跨章节翻页、延迟 CFI、状态机、内存警告、预加载、位置恢复等

Sources/RDReaderView/EPUBUI/TextPage/RDEPUBTextContentView.swift

@@ -107,6 +107,13 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate {
         return label
     }()
 
+    private let loadingSpinner: UIActivityIndicatorView = {
+        let spinner = UIActivityIndicatorView(style: .medium)
+        spinner.hidesWhenStopped = true
+        spinner.accessibilityIdentifier = "epub.reader.loadingSpinner"
+        return spinner
+    }()
+
     private lazy var longPressGestureRecognizer: UILongPressGestureRecognizer = {
         let gesture = UILongPressGestureRecognizer(target: self, action: #selector(handleLongPress(_:)))
         gesture.minimumPressDuration = 0.5
@@ -136,6 +143,7 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate {
 #endif
         addSubview(overlayView)
         addSubview(pageNumberLabel)
+        addSubview(loadingSpinner)
         addSubview(selectionLoupeView)
         addGestureRecognizer(longPressGestureRecognizer)
         addGestureRecognizer(panGestureRecognizer)
@@ -214,6 +222,7 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate {
             width: labelSize.width,
             height: labelSize.height
         )
+        loadingSpinner.center = CGPoint(x: bounds.midX, y: bounds.midY)
     }
 
     func configure(
@@ -226,6 +235,7 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate {
         highlights: [RDEPUBHighlight] = [],
         searchState: RDEPUBSearchState? = nil
     ) {
+        loadingSpinner.stopAnimating()
         currentPage = page
         currentChapterCFIMap = chapterCFIMap
         currentChapterFragmentOffsets = chapterFragmentOffsets
@@ -297,6 +307,44 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate {
         setNeedsLayout()
     }
 
+    func configureLoading(
+        pageNumber: Int,
+        totalPages: Int,
+        configuration: RDEPUBReaderConfiguration
+    ) {
+        currentPage = nil
+        currentChapterCFIMap = nil
+        currentChapterFragmentOffsets = [:]
+        currentSelection = nil
+        menuSelection = nil
+        activeSelectionHandle = nil
+        updateSelectionInteractionState(.idle)
+        currentHighlights = []
+        currentSearchState = nil
+        selectionController.clearSelection(renderView: coreTextRenderView)
+        contentInsets = configuration.reflowableContentInsets
+        backgroundColor = configuration.theme.contentBackgroundColor
+        pageNumberLabel.textColor = configuration.theme.contentTextColor
+        pageNumberLabel.text = "\(pageNumber) / \(totalPages)"
+        coverImageView.isHidden = true
+        coverImageView.image = nil
+
+#if canImport(DTCoreText)
+        coreTextContentView.isHidden = false
+        coreTextContentView.backgroundColor = .clear
+        coreTextContentView.attributedDisplayContent = nil
+        coreTextContentView.layoutFrame = nil
+        coreTextDisplayContent = nil
+        coreTextDisplayRange = nil
+#endif
+
+        overlayView.clearSelection()
+        backgroundOverlayView.clearSelection()
+        delegate?.textContentView(self, didChangeSelection: nil)
+        loadingSpinner.startAnimating()
+        setNeedsLayout()
+    }
+
     func clearSelection() {
         currentSelection = nil
         menuSelection = nil