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)

ReadViewDemo/ReadViewDemoUITests/Helpers/XCUIApplication+Launch.swift

@@ -5,18 +5,30 @@ extension XCUIApplication {
         bookTitleQuery: String = "回归验证样本",
         displayType: String? = nil,
         pageNumber: Int? = nil,
-        resetsReaderState: Bool = true
+        resetsReaderState: Bool = true,
+        windowSize: Int? = nil,
+        concurrency: Int? = nil,
+        clearsCache: Bool = false
     ) {
         var args = ["--demo-book-title", bookTitleQuery]
         if resetsReaderState {
             args.append("--demo-reset-state")
         }
+        if clearsCache {
+            args.append("--demo-clear-cache")
+        }
         if let displayType {
             args += ["--demo-display-type", displayType]
         }
         if let pageNumber {
             args += ["--demo-page", "\(pageNumber)"]
         }
+        if let windowSize {
+            args += ["--demo-window-size", "\(windowSize)"]
+        }
+        if let concurrency {
+            args += ["--demo-concurrency", "\(concurrency)"]
+        }
         launchArguments = args
         launch()
     }