refactor: move highlight menu logic from Demo to SDK library

Add RDPDFReaderPageView as base page view class in SDK:
- Handles all highlight menu interactions and display
- Provides RDPDFReaderPageViewDelegate for menu action callbacks
- Unifies with EPUB architecture: RDEPUBTextContentView pattern
- Reduces code duplication for applications using the SDK

Benefits:
- Reusable menu interaction logic across all PDF apps
- Consistent architecture with EPUB reader
- Single responsibility principle (UI in library, persistence in app)
- Backward compatible (existing code continues to work)

Applications now implement RDPDFReaderPageViewDelegate to:
- Respond to highlight menu actions
- Handle data persistence
- Refresh UI after changes

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Sources/RDPDFReaderView/Sources/RDPDFReaderPageView.swift

@@ -0,0 +1,118 @@
+import UIKit
+
+/// PDF 页面视图的基础类，提供统一的高亮菜单交互和文本选择菜单。
+/// 与 EPUB 的 RDEPUBTextContentView 对应。
+open class RDPDFReaderPageView: UIView, UIGestureRecognizerDelegate {
+
+    public weak var delegate: RDPDFReaderPageViewDelegate?
+
+    /// 宿主在页面视图创建时设置，用于显示真实内容（图片、PDF等）。
+    public let contentView = UIView()
+
+    /// 文本选择和高亮管理层，与 contentView 在同一坐标系。
+    public let textLayer = RDPDFReaderImageTextLayerView()
+
+    private var tappedHighlight: RDPDFReaderAnnotation?
+
+    public override init(frame: CGRect) {
+        super.init(frame: frame)
+        setupViews()
+    }
+
+    public required init?(coder: NSCoder) {
+        super.init(coder: coder)
+        setupViews()
+    }
+
+    private func setupViews() {
+        addSubview(contentView)
+        contentView.frame = bounds
+        contentView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
+
+        textLayer.frame = contentView.bounds
+        textLayer.autoresizingMask = [.flexibleWidth, .flexibleHeight]
+        contentView.addSubview(textLayer)
+
+        let tapGesture = UITapGestureRecognizer(target: self, action: #selector(handleTap(_:)))
+        addGestureRecognizer(tapGesture)
+    }
+
+    @objc private func handleTap(_ gesture: UITapGestureRecognizer) {
+        let point = gesture.location(in: textLayer)
+
+        if let highlight = highlightAt(point) {
+            showExistingHighlightMenu(for: highlight, at: gesture.location(in: self))
+            return
+        }
+    }
+
+    private func highlightAt(_ point: CGPoint) -> RDPDFReaderAnnotation? {
+        let currentPageAnnotations = textLayer.annotations.filter { $0.pageIndex == textLayer.pageIndex }
+        for annotation in currentPageAnnotations {
+            for rect in annotation.normalizedRects {
+                if rect.contains(point) {
+                    return annotation
+                }
+            }
+        }
+        return nil
+    }
+
+    private func showExistingHighlightMenu(for annotation: RDPDFReaderAnnotation, at point: CGPoint) {
+        tappedHighlight = annotation
+        becomeFirstResponder()
+
+        var menuItems: [UIMenuItem] = [
+            UIMenuItem(title: "复制", action: #selector(rd_copy_highlight(_:)))
+        ]
+
+        if annotation.note?.isEmpty == false {
+            menuItems.append(UIMenuItem(title: "删除注释", action: #selector(rd_delete_annotation(_:))))
+        } else {
+            menuItems.append(UIMenuItem(title: "注释", action: #selector(rd_annotate_highlight(_:))))
+        }
+
+        menuItems.append(UIMenuItem(title: "删除划线", action: #selector(rd_delete_highlight(_:))))
+
+        let menuController = UIMenuController.shared
+        menuController.menuItems = menuItems
+        menuController.setTargetRect(CGRect(x: point.x, y: point.y, width: 1, height: 1), in: self)
+        menuController.setMenuVisible(true, animated: true)
+    }
+
+    override open var canBecomeFirstResponder: Bool { true }
+
+    override open func canPerformAction(_ action: Selector, withSender sender: Any?) -> Bool {
+        if tappedHighlight != nil {
+            return action == #selector(rd_copy_highlight(_:))
+                || action == #selector(rd_delete_highlight(_:))
+                || action == #selector(rd_annotate_highlight(_:))
+                || action == #selector(rd_delete_annotation(_:))
+        }
+        return false
+    }
+
+    @objc private func rd_copy_highlight(_ sender: Any?) {
+        guard let highlight = tappedHighlight else { return }
+        delegate?.pageView(self, didRequestHighlightMenuAction: .copy, highlight: highlight)
+        tappedHighlight = nil
+    }
+
+    @objc private func rd_annotate_highlight(_ sender: Any?) {
+        guard let highlight = tappedHighlight else { return }
+        delegate?.pageView(self, didRequestHighlightMenuAction: .annotate, highlight: highlight)
+        tappedHighlight = nil
+    }
+
+    @objc private func rd_delete_annotation(_ sender: Any?) {
+        guard let highlight = tappedHighlight else { return }
+        delegate?.pageView(self, didRequestHighlightMenuAction: .deleteAnnotation, highlight: highlight)
+        tappedHighlight = nil
+    }
+
+    @objc private func rd_delete_highlight(_ sender: Any?) {
+        guard let highlight = tappedHighlight else { return }
+        delegate?.pageView(self, didRequestHighlightMenuAction: .deleteUnderline, highlight: highlight)
+        tappedHighlight = nil
+    }
+}