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/TextPage/RDEPUBPageInteractionController.swift

@@ -27,6 +27,12 @@ final class RDEPUBPageInteractionController {
 
     // MARK: - Hit Testing
 
+    /// 将视图坐标系的触摸点转为字符索引（对齐 WXRead 的 stringIndexForPoint:）
+    func characterIndexForViewPoint(at viewPoint: CGPoint, in view: UIView) -> Int? {
+        let localPoint = CGPoint(x: viewPoint.x, y: viewPoint.y)
+        return characterIndex(at: localPoint)
+    }
+
     func characterIndex(at point: CGPoint) -> Int? {
         guard let snapshot else { return nil }