refactor(reader): 重构高亮选区绘制架构，对齐 WXRead 实现方案

- 移除 RDEPUBSelectableTextView，改用原生 UITextView
- 新增 NSAttributedString 自定义属性（com.rdreader.highlight/underline）注入高亮
- RDEPUBTextPageRenderView 统一绘制高亮背景、文字和选区
- RDEPUBTextSelectionController 精简，选区矩形传递给 RenderView 绘制
- 新增高亮选区复刻 WXRead 实现方案文档
- UI 测试适配新架构

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Sources/RDReaderView/EPUBUI/RDEPUBReaderController+ContentDelegates.swift

@@ -83,7 +83,7 @@ extension RDEPUBReaderController: RDEPUBTextContentViewDelegate {
             updateCurrentSelection(nil)
             return
         }
-        updateCurrentSelection(normalizedTextSelection(selection))
+        updateCurrentSelection(selection)
     }
 
     /// 文本内容视图选区菜单操作回调
@@ -92,35 +92,10 @@ extension RDEPUBReaderController: RDEPUBTextContentViewDelegate {
         didRequestSelectionAction action: RDEPUBAnnotationMenuAction,
         selection: RDEPUBSelection?
     ) {
-        let normalizedSelection = selection.flatMap(normalizedTextSelection)
-        handleSelectionMenuAction(action, selection: normalizedSelection ?? currentSelection)
+        handleSelectionMenuAction(action, selection: selection ?? currentSelection)
         contentView.clearSelection()
     }
 
-    /// 标准化文本选区的偏移范围，确保在有效内容长度内
-    private func normalizedTextSelection(_ selection: RDEPUBSelection) -> RDEPUBSelection? {
-        guard let textBook,
-              let chapterData = textBook.chapterData(for: selection.location.href) else {
-            return scopedSelection(selection, relativeToSpineIndex: nil)
-        }
-        guard let payload = RDEPUBTextOffsetRangeInfo.decode(from: selection.rangeInfo) else {
-            return scopedSelection(selection, relativeToSpineIndex: nil)
-        }
-
-        let contentLength = max(chapterData.attributedContent.length, 1)
-        let lastInclusiveOffset = max(contentLength - 1, 1)
-        let start = max(0, min(payload.start, lastInclusiveOffset))
-        let endExclusive = max(start + 1, min(payload.end, contentLength))
-        let absoluteRange = NSRange(location: start, length: endExclusive - start)
-        let location = chapterData.location(for: absoluteRange, bookIdentifier: currentBookIdentifier)
-        return RDEPUBSelection(
-            bookIdentifier: currentBookIdentifier,
-            location: location,
-            text: selection.text,
-            rangeInfo: selection.rangeInfo,
-            createdAt: selection.createdAt
-        )
-    }
 
     /// 根据 EPUB 位置计算对应的页码
     func pageNumber(for location: RDEPUBLocation) -> Int? {