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/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
+    }
+}