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/RDEPUBChapterWindowSnapshot.swift

@@ -1,7 +1,7 @@
 import Foundation
 
 struct RDEPUBChapterWindowSnapshot {
-    /// 窗口中的章节（有序：prev, current, next）
+    /// 窗口中的章节（有序）
     let chapters: [RDEPUBRuntimeChapter]
 
     /// 展平后的连续页数组（供 RDReaderView 消费）
@@ -22,39 +22,26 @@ struct RDEPUBChapterWindowSnapshot {
 
     /// 从章节窗口构建快照
     static func from(
-        currentChapter: RDEPUBRuntimeChapter,
-        previousChapter: RDEPUBRuntimeChapter?,
-        nextChapter: RDEPUBRuntimeChapter?
+        chapters: [RDEPUBRuntimeChapter],
+        anchorSpineIndex: Int
     ) -> RDEPUBChapterWindowSnapshot {
-        var chapters: [RDEPUBRuntimeChapter] = []
-        var anchorIndex = 0
-        var pageOffset = 0
-
-        if let prev = previousChapter {
-            chapters.append(prev)
-            anchorIndex = 1
-            pageOffset = prev.pages.count
-        }
-
-        chapters.append(currentChapter)
-
-        if let next = nextChapter {
-            chapters.append(next)
-        }
+        let sortedChapters = chapters.sorted { $0.spineIndex < $1.spineIndex }
+        let anchorIndex = sortedChapters.firstIndex { $0.spineIndex == anchorSpineIndex } ?? 0
+        let pageOffset = sortedChapters.prefix(anchorIndex).reduce(0) { $0 + $1.pages.count }
 
         // 展平页数组
         var allPages: [RDEPUBTextPage] = []
-        for (chIdx, ch) in chapters.enumerated() {
+        for (chIdx, ch) in sortedChapters.enumerated() {
             for var page in ch.pages {
                 page.chapterIndex = chIdx
                 allPages.append(page)
             }
         }
 
-        let windowStartSpineIndex = chapters.first?.spineIndex ?? currentChapter.spineIndex
+        let windowStartSpineIndex = sortedChapters.first?.spineIndex ?? anchorSpineIndex
 
         return RDEPUBChapterWindowSnapshot(
-            chapters: chapters,
+            chapters: sortedChapters,
             flattenedPages: allPages,
             anchorChapterIndex: anchorIndex,
             anchorPageOffset: pageOffset,
@@ -83,4 +70,4 @@ struct RDEPUBChapterWindowSnapshot {
 
     /// 总页数（窗口内）
     var pageCount: Int { flattenedPages.count }
-}
+}