长章节内存优化 P1：章节级共享显示内容与 layouter，高亮转 overlay

- 新增 RDEPUBChapterDisplayContentCache（LRU 2，主线程限定，控制器
  持有）：整章显示串构建时一次性注入全章主题色与暗黑图替换，配套
  共享 DTCoreTextLayouter；签名 = 章节内容对象标识 + 主题双色 +
  暗黑图配置，设置/主题变化自动失效重建
- RDEPUBTextContentView.configure 改为引用共享 entry，删除每次翻页
  的整章可变拷贝与按页属性写入
- 高亮/下划线改走 overlay decoration（高亮背景层、下划线前景层），
  移除 applyHighlightsToContent 与 render view 的属性绘制路径，
  高亮增删不再触发整章重建
- 内存警告时清空 display cache；shouldAvoidReaderPageCaching 保持
  保守（P1-4 待真机数据）

验证：编译通过；高亮/批注/选区/翻页/设置 13 项 UI 回归全过。
SearchTests 10 项失败经二分确认为先存回归（5a41066 与 e4e629a 均
复现，最后已知通过为 2026-06-08），与本次改动无关，待单独排查。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Sources/RDReaderView/EPUBUI/TextPage/RDEPUBTextPageRenderView.swift

@@ -22,12 +22,6 @@ final class RDEPUBTextPageRenderView: UIView {
         }
     }
 
-    var attributedDisplayContent: NSAttributedString? {
-        didSet {
-            invalidateStaticContent()
-        }
-    }
-
     var selectionRects: [CGRect] = [] {
         didSet {
             setNeedsDisplay()
@@ -78,9 +72,6 @@ final class RDEPUBTextPageRenderView: UIView {
         if let cachedStaticImage {
             cachedStaticImage.draw(in: bounds)
         } else {
-            if let attributedDisplayContent {
-                drawHighlights(in: context, attributedString: attributedDisplayContent, layoutFrame: layoutFrame)
-            }
             layoutFrame.draw(in: context, options: drawOptions)
         }
 
@@ -96,59 +87,6 @@ final class RDEPUBTextPageRenderView: UIView {
         }
     }
 
-    private func drawHighlights(
-        in context: CGContext,
-        attributedString: NSAttributedString,
-        layoutFrame: DTCoreTextLayoutFrame
-    ) {
-        let fullRange = NSRange(location: 0, length: attributedString.length)
-
-        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)
-            }
-        }
-
-        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()
-            }
-        }
-    }
-
-    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
-    }
-
     private func drawSelection(in context: CGContext) {
         guard !selectionRects.isEmpty else { return }
         selectionColor.setFill()
@@ -261,9 +199,6 @@ final class RDEPUBTextPageRenderView: UIView {
         let renderer = UIGraphicsImageRenderer(size: bounds.size, format: format)
         return renderer.image { _ in
             guard let staticContext = UIGraphicsGetCurrentContext() else { return }
-            if let attributedDisplayContent {
-                drawHighlights(in: staticContext, attributedString: attributedDisplayContent, layoutFrame: layoutFrame)
-            }
             layoutFrame.draw(in: staticContext, options: drawOptions)
         }
     }