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

ReadViewDemo/ReadViewDemo.xcodeproj/project.pbxproj

@@ -17,6 +17,7 @@
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@@ -51,6 +52,7 @@
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@@ -190,6 +192,7 @@
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 				1A2B3C4D00000013AABBCC01 /* FanrenParseTimeTest.swift */,
 				CFE01DBDCB8D790832A4DE3F /* SearchTests.swift */,
+				1A2B3C4D00000016AABBCC01 /* AsyncChapterLoadingTests.swift */,
 			);
 			path = ReaderUITests;
 			sourceTree = "<group>";
@@ -354,6 +357,7 @@
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 				1A2B3C4D00000014AABBCC01 /* FanrenParseTimeTest.swift in Sources */,
 				369C9658D870DCAFC17EB7F7 /* SearchTests.swift in Sources */,
+				1A2B3C4D00000015AABBCC01 /* AsyncChapterLoadingTests.swift in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};