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/EPUBTextRendering/RDEPUBChapterData.swift

@@ -297,6 +297,33 @@ public final class RDEPUBChapterData {
         tableOfContentsItems(from: items, normalizer: normalizer).first
     }
 
+    // MARK: - 高亮属性注入（对齐 WXRead 的 WRChapterData.addHighlightInRange:key:itemId:color:）
+
+    /// 将高亮/下划线作为自定义属性注入 NSAttributedString，
+    /// 供 CoreText 渲染时读取绘制（对齐 WXRead 的 com.weread.highlight / com.weread.underline）
+    public func applyHighlights(
+        to content: NSMutableAttributedString,
+        page: RDEPUBTextPage,
+        highlights: [RDEPUBHighlight]
+    ) {
+        for highlight in highlights {
+            guard highlight.location.href == chapter.href else { continue }
+            guard let range = absoluteRange(for: highlight) else { continue }
+            let overlap = NSIntersectionRange(range, page.contentRange)
+            guard overlap.length > 0 else { continue }
+            let relativeRange = NSRange(location: overlap.location - page.pageStartOffset, length: overlap.length)
+            guard relativeRange.location >= 0,
+                  relativeRange.location + relativeRange.length <= content.length else { continue }
+
+            switch highlight.style {
+            case .highlight:
+                content.addAttribute(kRDEPUBHighlightAttributeName, value: highlight.uiColor, range: relativeRange)
+            case .underline:
+                content.addAttribute(kRDEPUBUnderlineAttributeName, value: highlight.uiColor, range: relativeRange)
+            }
+        }
+    }
+
     // MARK: - 私有工具方法
 
     /// 将页面的起止偏移量转换为半开区间 [start, end+1)