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/RDEPUBTextPageRenderView.swift

@@ -12,19 +12,19 @@ final class RDEPUBTextPageRenderView: UIView {
 
     var layoutFrame: DTCoreTextLayoutFrame? {
         didSet {
-            setNeedsDisplay()
+            invalidateStaticContent()
         }
     }
 
     var drawOptions: DTCoreTextLayoutFrameDrawingOptions = DTCoreTextLayoutFrameDrawingOptions(rawValue: 1)! {
         didSet {
-            setNeedsDisplay()
+            invalidateStaticContent()
         }
     }
 
     var attributedDisplayContent: NSAttributedString? {
         didSet {
-            setNeedsDisplay()
+            invalidateStaticContent()
         }
     }
 
@@ -44,6 +44,12 @@ final class RDEPUBTextPageRenderView: UIView {
 
     private let selectionHandleHitSlop: CGFloat = 20
 
+    private var cachedStaticImage: UIImage?
+
+    private var cachedStaticBoundsSize: CGSize = .zero
+
+    private var needsStaticContentRedraw = true
+
     override init(frame: CGRect) {
         super.init(frame: frame)
         backgroundColor = .clear
@@ -61,17 +67,35 @@ final class RDEPUBTextPageRenderView: UIView {
 
         context.saveGState()
 
-        if let attributedDisplayContent {
-            drawHighlights(in: context, attributedString: attributedDisplayContent, layoutFrame: layoutFrame)
+        if cachedStaticImage == nil
+            || cachedStaticBoundsSize != bounds.size
+            || needsStaticContentRedraw {
+            cachedStaticImage = renderStaticImage(layoutFrame: layoutFrame)
+            cachedStaticBoundsSize = bounds.size
+            needsStaticContentRedraw = false
         }
 
-        layoutFrame.draw(in: context, options: drawOptions)
+        if let cachedStaticImage {
+            cachedStaticImage.draw(in: bounds)
+        } else {
+            if let attributedDisplayContent {
+                drawHighlights(in: context, attributedString: attributedDisplayContent, layoutFrame: layoutFrame)
+            }
+            layoutFrame.draw(in: context, options: drawOptions)
+        }
 
         drawSelection(in: context)
 
         context.restoreGState()
     }
 
+    override func layoutSubviews() {
+        super.layoutSubviews()
+        if cachedStaticBoundsSize != bounds.size {
+            invalidateStaticContent()
+        }
+    }
+
     private func drawHighlights(
         in context: CGContext,
         attributedString: NSAttributedString,
@@ -223,6 +247,26 @@ final class RDEPUBTextPageRenderView: UIView {
 
         context.restoreGState()
     }
+
+    private func invalidateStaticContent() {
+        cachedStaticImage = nil
+        needsStaticContentRedraw = true
+        setNeedsDisplay()
+    }
+
+    private func renderStaticImage(layoutFrame: DTCoreTextLayoutFrame) -> UIImage? {
+        guard bounds.width > 0, bounds.height > 0 else { return nil }
+        let format = UIGraphicsImageRendererFormat.default()
+        format.opaque = false
+        let renderer = UIGraphicsImageRenderer(size: bounds.size, format: format)
+        return renderer.image { _ in
+            guard let staticContext = UIGraphicsGetCurrentContext() else { return }
+            if let attributedDisplayContent {
+                drawHighlights(in: staticContext, attributedString: attributedDisplayContent, layoutFrame: layoutFrame)
+            }
+            layoutFrame.draw(in: staticContext, options: drawOptions)
+        }
+    }
 }
 
 private extension CGPoint {