feat: configurable chapter window & parallel metadata parsing with benchmark

1. Configurable chapter window size (onDemandChapterWindowSize: 3-15)
   - Parameterized window radius in RDEPUBChapterRuntimeStore
   - Updated RDEPUBChapterWindowCoordinator to use configurable radius
   - RDEPUBChapterWindowSnapshot.from() accepts chapter array instead of fixed prev/next
   - Even numbers round up to odd (4→5), min 3, max 15

2. Configurable metadata parsing concurrency (metadataParsingConcurrency)
   - Default equals CPU core count
   - Parallel execution via OperationQueue in paginateMetadataOnly
   - Each worker creates independent builder instance
   - NSLock protects result aggregation

3. Per-chapter and total wall-clock timing instrumentation
   - Separated render vs I/O timing per chapter
   - Summary log with wallClockMs, renderTotalMs, writeTotalMs, avgRenderMs
   - Timing stored in RDEPUBReaderContext for test access

4. UI automation test infrastructure
   - Added --demo-window-size, --demo-concurrency, --demo-clear-cache launch args
   - DemoReaderState exposes windowSize, parseMs, parseConcurrency
   - ConfigurableWindowTests: 5 test cases for window size 3/5/15
   - ConcurrentParsingTests: 4 test cases for concurrency 2/4
   - MetadataParseBenchmarkTests: serial vs parallel benchmark

5. Bug fixes
   - Fixed page snap-back during background parsing (isUserInteracting check)
   - Reduced BookPageMap refresh frequency from 16 to 32 chapters
   - Moved waitForReadingInteractionToSettle outside operation loop

6. Design doc: dual-layer PageMap (estimated + precise mixed)

Sources/RDReaderView/EPUBUI/Settings/RDEPUBReaderConfiguration.swift

@@ -107,6 +107,12 @@ public struct RDEPUBReaderConfiguration: Equatable {
     public var fixedLayoutSpreadMode: RDEPUBFixedLayoutSpreadMode
     /// 文本渲染引擎，默认使用 DTCoreText
     public var textRenderingEngine: RDEPUBTextRenderingEngine
+    /// 章节按需加载窗口大小（总章节数，包含当前章），默认 3，范围 3...15，偶数自动向上取奇
+    public var onDemandChapterWindowSize: Int
+    /// 后台元数据解析并发数，默认为 CPU 核心数。
+    /// 若 profiling 显示 renderTotalMs ≈ wallClockMs（渲染受限），维持核心数即可；
+    /// 若 writeTotalMs 占比显著（I/O 等待），可试探 cpuCount * 1.25~1.5 以填充 I/O 等待间隙。
+    public var metadataParsingConcurrency: Int
 
     // MARK: 初始化
 
@@ -128,6 +134,8 @@ public struct RDEPUBReaderConfiguration: Equatable {
     ///   - fixedLayoutFit: 固定布局适配模式
     ///   - fixedLayoutSpreadMode: 固定布局跨页模式
     ///   - textRenderingEngine: 文本渲染引擎
+    ///   - onDemandChapterWindowSize: 章节按需加载窗口大小（总章节数 3...15，偶数自动向上取奇）
+    ///   - metadataParsingConcurrency: 后台元数据解析并发数，默认 CPU 核心数
     public init(
         fontSize: CGFloat = 15,
         lineHeightMultiple: CGFloat = 1.6,
@@ -146,7 +154,9 @@ public struct RDEPUBReaderConfiguration: Equatable {
         darkImageBlendRatio: CGFloat = 0.15,
         fixedLayoutFit: RDEPUBFixedLayoutFit = .page,
         fixedLayoutSpreadMode: RDEPUBFixedLayoutSpreadMode = .automatic,
-        textRenderingEngine: RDEPUBTextRenderingEngine = .dtCoreText
+        textRenderingEngine: RDEPUBTextRenderingEngine = .dtCoreText,
+        onDemandChapterWindowSize: Int = 3,
+        metadataParsingConcurrency: Int = ProcessInfo.processInfo.activeProcessorCount
     ) {
         self.fontSize = fontSize
         self.lineHeightMultiple = lineHeightMultiple
@@ -166,12 +176,25 @@ public struct RDEPUBReaderConfiguration: Equatable {
         self.fixedLayoutFit = fixedLayoutFit
         self.fixedLayoutSpreadMode = fixedLayoutSpreadMode
         self.textRenderingEngine = textRenderingEngine
+        self.onDemandChapterWindowSize = Self.normalizedChapterWindowSize(onDemandChapterWindowSize)
+        self.metadataParsingConcurrency = max(1, metadataParsingConcurrency)
     }
 
     /// 默认配置实例，使用所有参数的默认值
     public static let `default` = RDEPUBReaderConfiguration()
 }
 
+extension RDEPUBReaderConfiguration {
+    static func normalizedChapterWindowSize(_ size: Int) -> Int {
+        let clamped = max(3, min(15, size))
+        return clamped % 2 == 0 ? clamped + 1 : clamped
+    }
+
+    var chapterWindowRadius: Int {
+        onDemandChapterWindowSize / 2
+    }
+}
+
 // MARK: - 配置转换
 
 extension RDEPUBReaderConfiguration {