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),

Sources/RDReaderView/EPUBUI/RDEPUBReaderController+DataSource.swift

@@ -345,6 +345,7 @@ extension RDEPUBReaderController: RDReaderDataSource, RDReaderPageProvider, RDRe
     }
 
     public func pageNum(readerView: RDReaderView, pageNum: Int) {
+        RDEPUBMemoryProbe.logPageTurn()
         readerContext.markUserNavigationActivity()
         updateCurrentSelection(nil)
         reconcileTextPaginationSizeIfNeeded(for: pageNum)

Sources/RDReaderView/EPUBUI/RDEPUBReaderController.swift

@@ -297,6 +297,9 @@ public final class RDEPUBReaderController: UIViewController {
     public override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
         super.viewWillTransition(to: size, with: coordinator)
         runtime.viewportMonitor.viewWillTransition(with: coordinator)
+        coordinator.animate(alongsideTransition: nil) { _ in
+            RDEPUBMemoryProbe.log("orientationTransition size=\(Int(size.width))x\(Int(size.height))")
+        }
     }
 
     func showSearchBar() {

Sources/RDReaderView/EPUBUI/ReaderController/ChapterRuntime/RDEPUBChapterLoader.swift

@@ -491,7 +491,6 @@ final class RDEPUBChapterLoader {
     ) -> [RDEPUBTextPage] {
         let totalPageCount = pageRanges.count
         return pageRanges.enumerated().map { (pageIndex, range) in
-            let pageContent = typesetString.attributedSubstring(from: range)
             let metadata: RDEPUBTextPageMetadata
             if let metaList = metadataSource, pageIndex < metaList.count {
 
@@ -513,7 +512,6 @@ final class RDEPUBChapterLoader {
                 pageIndexInChapter: pageIndex,
                 totalPagesInChapter: totalPageCount,
                 chapterContent: typesetString,
-                content: pageContent,
                 contentRange: range,
                 pageStartOffset: range.location,
                 pageEndOffset: range.location + range.length - 1,

Sources/RDReaderView/EPUBUI/ReaderController/ChapterRuntime/RDEPUBChapterRuntimeStore.swift

@@ -62,6 +62,7 @@ final class RDEPUBChapterRuntimeStore {
 
     func insertChapter(_ chapter: RDEPUBRuntimeChapter) {
         chapterDataCache[chapter.spineIndex] = chapter
+        RDEPUBMemoryProbe.log("chapterLoaded spine=\(chapter.spineIndex) pages=\(chapter.pages.count)")
     }
 
     func insertPageCount(_ pc: RDEPUBRuntimePageCount, for key: RDEPUBChapterCacheKey) {
@@ -188,6 +189,7 @@ final class RDEPUBChapterRuntimeStore {
     }
 
     func invalidateAllForSettingsChange() {
+        RDEPUBMemoryProbe.log("settingsChangeInvalidateAll")
         chapterDataCache.removeAll()
         pageCountCache.removeAll()
         imageCache.removeAllObjects()

Sources/RDReaderView/EPUBUI/ReaderController/RDEPUBMemoryProbe.swift

@@ -0,0 +1,43 @@
+import Foundation
+
+/// Memory footprint probe for the long-chapter optimization work
+/// (Doc/LONG_CHAPTER_MEMORY_OPTIMIZATION_PLAN.md, P0-2). Enabled with the
+/// `--demo-memory-probe` launch argument; logs phys_footprint at chapter
+/// load, every 20 page turns, settings invalidation, and rotation.
+enum RDEPUBMemoryProbe {
+
+    static let isEnabled = ProcessInfo.processInfo.arguments.contains("--demo-memory-probe")
+
+    private static let pageTurnLogStride = 20
+
+    /// Main-thread only (page turns are delivered on main).
+    private static var pageTurnCount = 0
+
+    static func logPageTurn() {
+        guard isEnabled else { return }
+        pageTurnCount += 1
+        guard pageTurnCount % pageTurnLogStride == 0 else { return }
+        log("pageTurn count=\(pageTurnCount)")
+    }
+
+    static func log(_ event: String) {
+        guard isEnabled else { return }
+        let megabytes = Double(currentFootprint()) / 1_048_576
+        print(String(format: "[EPUB][MemoryProbe] %@ footprint=%.1fMB", event, megabytes))
+    }
+
+    /// phys_footprint matches the value Xcode's memory gauge and Jetsam use.
+    private static func currentFootprint() -> UInt64 {
+        var info = task_vm_info_data_t()
+        var count = mach_msg_type_number_t(
+            MemoryLayout<task_vm_info_data_t>.size / MemoryLayout<integer_t>.size
+        )
+        let result = withUnsafeMutablePointer(to: &info) { pointer in
+            pointer.withMemoryRebound(to: integer_t.self, capacity: Int(count)) {
+                task_info(mach_task_self_, task_flavor_t(TASK_VM_INFO), $0, &count)
+            }
+        }
+        guard result == KERN_SUCCESS else { return 0 }
+        return info.phys_footprint
+    }
+}

Sources/RDReaderView/EPUBUI/TextPage/RDEPUBTextContentView.swift

@@ -322,6 +322,9 @@ final class RDEPUBTextContentView: UIView, UIGestureRecognizerDelegate, RDReader
         // layouter alive for the visible page. Avoid reader-level page
         // caching so preloading does not retain several full-chapter
         // display copies at once.
+        // Do NOT relax this until chapter-level shared display content
+        // lands (LONG_CHAPTER_MEMORY_OPTIMIZATION_PLAN.md P1-1): cached
+        // pages would each pin their own full-chapter copy again.
         return currentPage == nil
             || loadingSpinner.isAnimating
             || coreTextDisplayContent != nil