feat: complete large-book pagination runtime and UI coverage

Sources/RDReaderView/EPUBUI/ReaderController/ChapterRuntime/RDEPUBChapterOffsetMap.swift

@@ -0,0 +1,22 @@
+import Foundation
+
+struct RDEPUBChapterOffsetMap {
+    let fragmentOffsets: [String: Int]
+    let pageStartOffsets: [Int]
+    let pageEndOffsets: [Int]
+
+    /// fragmentID -> 章内字符偏移
+    func chapterOffset(forFragmentID fragmentID: String) -> Int? {
+        return fragmentOffsets[fragmentID]
+    }
+
+    /// 章内字符偏移 -> 章内页码（从 0 开始）
+    func pageIndex(forChapterOffset offset: Int) -> Int? {
+        for i in 0..<pageStartOffsets.count {
+            if offset >= pageStartOffsets[i] && offset <= pageEndOffsets[i] {
+                return i
+            }
+        }
+        return nil
+    }
+}