修复 EPUB 文本分页显示错位并补充分页调试能力

本次提交围绕 DTCoreText 文本页的分页一致性、交互索引和高亮命中进行了集中修复。

主要改动：

1. 文本页显示改为基于整章 attributed string 的上下文布局，只对当前页 range 进行渲染，避免页面子串重新换行导致的页末断行偏差。

2. 页面布局快照与交互控制器统一改为使用 chapter-absolute 索引，修正点击、选区、菜单锚点与高亮矩形在整章上下文下的定位。

3. 修复跨章节高亮串页问题，并调整文本页高亮命中逻辑：保留 CoreText 层绘制，点击时按高亮真实 rect 精确命中，避免重复绘制和整行误判。

4. 收紧 reader 级页面缓存策略，避免预加载同时持有多份整章显示副本带来的内存放大。

5. 新增分页边界校验器、垂直对齐器和 settings-flip 自动化调试入口，用于复现与诊断页范围/显示度量不一致问题。

6. 放宽 inline attachment 的 avoid-break 处理，并补充相关分页问题调查文档与索引。

Sources/RDReaderView/EPUBUI/TextPage/RDEPUBPageInteractionController.swift

@@ -44,10 +44,12 @@ final class RDEPUBPageInteractionController {
             x: point.x - line.baselineOrigin.x,
             y: point.y - line.baselineOrigin.y
         )
+        // The layout frame is built in chapter context, so DTCoreText string
+        // indices are chapter-absolute already.
         let idx = dtLine.stringIndex(forPosition: relativePoint)
         guard idx != NSNotFound, idx >= 0 else { return nil }
         return normalizedIndex(
-            idx + pageOffset(for: snapshot),
+            idx,
             lineRange: line.stringRange,
             pageRange: snapshot.pageContentRange
         )
@@ -74,10 +76,9 @@ final class RDEPUBPageInteractionController {
 
 #if canImport(DTCoreText)
             guard let dtLine = dtLineContaining(range: line.stringRange) else { continue }
-            let pageOffset = pageOffset(for: snapshot)
-            let startX = dtLine.offset(forStringIndex: overlap.location - pageOffset)
+            let startX = dtLine.offset(forStringIndex: overlap.location)
             let endIdx = overlap.location + overlap.length
-            let endX = dtLine.offset(forStringIndex: endIdx - pageOffset)
+            let endX = dtLine.offset(forStringIndex: endIdx)
 #else
             let startX: CGFloat = 0
             let endX: CGFloat = line.frame.width
@@ -133,7 +134,7 @@ final class RDEPUBPageInteractionController {
 
 #if canImport(DTCoreText)
         guard let dtLine = dtLineContaining(range: line.stringRange) else { return nil }
-        let offsetX = dtLine.offset(forStringIndex: index - pageOffset(for: snapshot))
+        let offsetX = dtLine.offset(forStringIndex: index)
 #else
         let offsetX: CGFloat = 0
 #endif
@@ -183,16 +184,11 @@ final class RDEPUBPageInteractionController {
 #if canImport(DTCoreText)
 
     private func dtLineContaining(range: NSRange) -> DTCoreTextLayoutLine? {
-        guard let dtLayoutFrame, let snapshot else { return nil }
-        let localLocation = max(range.location - pageOffset(for: snapshot), 0)
-        return dtLayoutFrame.lineContaining(UInt(localLocation))
+        guard let dtLayoutFrame else { return nil }
+        return dtLayoutFrame.lineContaining(UInt(max(range.location, 0)))
     }
 #endif
 
-    private func pageOffset(for snapshot: RDEPUBPageLayoutSnapshot) -> Int {
-        snapshot.page.pageStartOffset
-    }
-
     private func mergeAdjacentRects(_ rects: [CGRect]) -> [CGRect] {
         guard rects.count > 1 else { return rects }