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/ReaderController/ChapterRuntime/RDEPUBChapterRuntimeStore.swift

@@ -23,7 +23,7 @@ final class RDEPUBChapterRuntimeStore {
     /// 当前章 spineIndex
     private(set) var currentSpineIndex: Int?
 
-    /// 当前窗口内的 spineIndex 集合（当前 + prev + next）
+    /// 当前窗口内的 spineIndex 集合（以当前章为中心，按配置半径展开）
     private(set) var windowSpineIndices: [Int] = []
 
     // MARK: - 请求通道（前台导航 vs 后台预取，语义独立，互不抢占）
@@ -75,13 +75,17 @@ final class RDEPUBChapterRuntimeStore {
 
     // MARK: - 窗口管理
 
-    /// 设定当前章，自动计算 ±1 窗口
-    func setCurrentChapter(spineIndex: Int, totalSpineCount: Int) {
+    /// 设定当前章，自动计算按半径展开的窗口
+    func setCurrentChapter(spineIndex: Int, totalSpineCount: Int, windowRadius: Int = 1) {
         currentSpineIndex = spineIndex
-        var window = [spineIndex]
-        if spineIndex > 0 { window.append(spineIndex - 1) }
-        if spineIndex < totalSpineCount - 1 { window.append(spineIndex + 1) }
-        windowSpineIndices = window
+        let radius = max(0, windowRadius)
+        let lowerBound = max(0, spineIndex - radius)
+        let upperBound = min(totalSpineCount - 1, spineIndex + radius)
+        guard lowerBound <= upperBound else {
+            windowSpineIndices = [spineIndex]
+            return
+        }
+        windowSpineIndices = Array(lowerBound...upperBound)
     }
 
     /// 返回窗口外、应该淘汰的 spineIndex
@@ -183,4 +187,4 @@ final class RDEPUBChapterRuntimeStore {
         pageCountCache.removeAll()
         imageCache.removeAllObjects()
     }
-}
+}