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/RDEPUBTextAnnotationOverlay.swift

@@ -32,7 +32,7 @@ final class RDEPUBTextAnnotationOverlay: RDEPUBSelectionOverlayView {
             case .highlight:
                 content.addAttribute(
                     .backgroundColor,
-                    value: UIColor(rdHexString: highlight.color, alpha: 0.45) ?? UIColor(red: 248 / 255, green: 225 / 255, blue: 108 / 255, alpha: 0.45),
+                    value: UIColor(rdHexString: highlight.color, alpha: 0.35) ?? UIColor(red: 248 / 255, green: 225 / 255, blue: 108 / 255, alpha: 0.35),
                     range: relativeRange
                 )
             case .underline:
@@ -128,8 +128,8 @@ final class RDEPUBTextAnnotationOverlay: RDEPUBSelectionOverlayView {
             let rects = interactionController.selectionRects(for: absoluteRange)
             guard !rects.isEmpty else { continue }
 
-            let color = UIColor(rdHexString: highlight.color, alpha: 0.45)
-                ?? UIColor(red: 248 / 255, green: 225 / 255, blue: 108 / 255, alpha: 0.45)
+            let color = UIColor(rdHexString: highlight.color, alpha: 0.35)
+                ?? UIColor(red: 248 / 255, green: 225 / 255, blue: 108 / 255, alpha: 0.35)
             let decoration = RDEPUBTextOverlayDecoration(
                 kind: highlight.style == .underline ? .underline : .highlight,
                 absoluteRange: absoluteRange,