长章节内存优化 P0：每页子串改按需构造并新增内存打点

- RDEPUBTextPage.content 由存储属性改为基于 chapterContent+contentRange
  的计算属性，三处构造点不再生成每页子串，章节缓存窗口内每章少一份
  整章文本常驻副本
- 新增 RDEPUBMemoryProbe（--demo-memory-probe 开启），在章节插入、
  每 20 次翻页、设置失效、转屏完成时输出 phys_footprint
- shouldAvoidReaderPageCaching 补注释，锁定放开前置条件为 P1-1
- 新增实施清单文档 LONG_CHAPTER_MEMORY_OPTIMIZATION_PLAN.md 并勾选 P0

验证：Demo 编译通过；UI 回归 9/9 通过（PageNavigation/
ReaderOpenClose/LargeBookOnDemand）

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

Sources/RDReaderView/EPUBTextRendering/BuildPipeline/RDEPUBTextBookBuilder.swift

@@ -291,7 +291,6 @@ public final class RDEPUBTextBookBuilder {
                 pageIndexInChapter: localPageIndex,
                 totalPagesInChapter: effectiveFrames.count,
                 chapterContent: chapterAttributedContent,
-                content: content.attributedSubstring(from: range),
                 contentRange: range,
                 pageStartOffset: range.location,
                 pageEndOffset: range.location + max(range.length - 1, 0),

Sources/RDReaderView/EPUBTextRendering/BuildPipeline/RDEPUBTextBookModels.swift

@@ -54,7 +54,15 @@ public struct RDEPUBTextPage: Equatable {
 
     public var chapterContent: NSAttributedString
 
-    public var content: NSAttributedString
+    /// Page substring derived on demand from `chapterContent` + `contentRange`.
+    /// Not stored: keeping a per-page substring alive roughly doubles the
+    /// chapter's resident text memory across the chapter cache window.
+    public var content: NSAttributedString {
+        let bounds = NSRange(location: 0, length: chapterContent.length)
+        let clamped = NSIntersectionRange(contentRange, bounds)
+        guard clamped.length > 0 else { return NSAttributedString() }
+        return chapterContent.attributedSubstring(from: clamped)
+    }
 
     public var contentRange: NSRange
 

Sources/RDReaderView/EPUBTextRendering/RDPlainTextBookBuilder.swift

@@ -75,7 +75,6 @@ public final class RDPlainTextBookBuilder {
                     pageIndexInChapter: localPageIndex,
                     totalPagesInChapter: effectiveFrames.count,
                     chapterContent: chapterAttributedContent,
-                    content: content.attributedSubstring(from: range),
                     contentRange: range,
                     pageStartOffset: range.location,
                     pageEndOffset: range.location + max(range.length - 1, 0),