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/RDEPUBTextPageRenderView.swift

@@ -5,6 +5,7 @@ import DTCoreText
 
 /// 基于 DTCoreText 的 Core Text 直接绘制视图
 /// 将 DTCoreText 的排版结果直接绘制到 UIView 上，跳过 UITextView 的间接渲染。
+/// 对齐 WXRead 的 WRPageView：在同一 drawRect 中绘制高亮背景、文字和选区。
 final class RDEPUBTextPageRenderView: UIView {
     var layoutFrame: DTCoreTextLayoutFrame? {
         didSet {
@@ -18,6 +19,27 @@ final class RDEPUBTextPageRenderView: UIView {
         }
     }
 
+    /// 用于高亮绘制的 attributed string（包含 com.rdreader.highlight 等自定义属性）
+    var attributedDisplayContent: NSAttributedString? {
+        didSet {
+            setNeedsDisplay()
+        }
+    }
+
+    // MARK: - 选区绘制（对齐 WXRead 的 _drawSelectionInContext:）
+
+    /// 选区矩形数组，由 RDEPUBTextSelectionController 设置
+    var selectionRects: [CGRect] = [] {
+        didSet {
+            setNeedsDisplay()
+        }
+    }
+
+    /// 选区颜色（蓝色半透明，对齐 WXRead）
+    var selectionColor: UIColor = UIColor(red: 70 / 255, green: 140 / 255, blue: 1, alpha: 0.24)
+
+    // MARK: - Init
+
     override init(frame: CGRect) {
         super.init(frame: frame)
         backgroundColor = .clear
@@ -29,13 +51,94 @@ final class RDEPUBTextPageRenderView: UIView {
         fatalError("init(coder:) has not been implemented")
     }
 
+    // MARK: - Draw
+
     override func draw(_ rect: CGRect) {
         guard let context = UIGraphicsGetCurrentContext(),
               let layoutFrame else { return }
 
         context.saveGState()
+
+        // 1. 绘制高亮背景（在文字下方，对齐 WXRead 的 drawHighlightsInContext:）
+        if let attributedDisplayContent {
+            drawHighlights(in: context, attributedString: attributedDisplayContent, layoutFrame: layoutFrame)
+        }
+
+        // 2. 绘制文字
         layoutFrame.draw(in: context, options: drawOptions)
+
+        // 3. 绘制选区（在文字上方）
+        drawSelection(in: context)
+
         context.restoreGState()
     }
+
+    // MARK: - 高亮绘制（对齐 WXRead 的 WRCoreTextLayoutFrame.drawHighlightsInContext:）
+
+    private func drawHighlights(
+        in context: CGContext,
+        attributedString: NSAttributedString,
+        layoutFrame: DTCoreTextLayoutFrame
+    ) {
+        let fullRange = NSRange(location: 0, length: attributedString.length)
+
+        // 绘制高亮背景
+        attributedString.enumerateAttribute(kRDEPUBHighlightAttributeName, in: fullRange) { value, range, _ in
+            guard let color = value as? UIColor else { return }
+            let rects = computeHighlightRects(for: range, layoutFrame: layoutFrame)
+            color.setFill()
+            for rect in rects {
+                context.fill(rect)
+            }
+        }
+
+        // 绘制下划线
+        attributedString.enumerateAttribute(kRDEPUBUnderlineAttributeName, in: fullRange) { value, range, _ in
+            guard let color = value as? UIColor else { return }
+            let rects = computeHighlightRects(for: range, layoutFrame: layoutFrame)
+            color.setStroke()
+            context.setLineWidth(2)
+            for rect in rects {
+                let y = rect.maxY - 1
+                context.move(to: CGPoint(x: rect.minX, y: y))
+                context.addLine(to: CGPoint(x: rect.maxX, y: y))
+                context.strokePath()
+            }
+        }
+    }
+
+    /// 计算指定字符范围的视觉矩形（对齐 WXRead 的 rectsForRange: 算法）
+    private func computeHighlightRects(for range: NSRange, layoutFrame: DTCoreTextLayoutFrame) -> [CGRect] {
+        var rects: [CGRect] = []
+        guard let lines = layoutFrame.lines as? [DTCoreTextLayoutLine] else { return rects }
+
+        for line in lines {
+            let lineRange = line.stringRange()
+            let overlap = NSIntersectionRange(range, lineRange)
+            guard overlap.length > 0 else { continue }
+
+            let startX = line.offset(forStringIndex: overlap.location)
+            let endX = line.offset(forStringIndex: overlap.location + overlap.length)
+            let rect = CGRect(
+                x: line.baselineOrigin.x + startX,
+                y: line.baselineOrigin.y - line.ascent,
+                width: endX - startX,
+                height: line.ascent + line.descent
+            )
+            rects.append(rect)
+        }
+
+        return rects
+    }
+
+    // MARK: - 选区绘制
+
+    private func drawSelection(in context: CGContext) {
+        guard !selectionRects.isEmpty else { return }
+        selectionColor.setFill()
+        for rect in selectionRects {
+            context.fill(rect)
+        }
+    }
 }
 #endif