fix: #34 #35 移除 contentHeightEstimate 80px 硬编码

- actualContentHeight 通过 GeometryReader 获取真实内容高度
- scrollProgress 使用实际高度计算，精度显著提升
- #36 ReadingEventCollector 已清理完毕（无 ObservableObject/Combine）

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

AIStudyApp/AIStudyApp/Features/MaterialReader/MaterialReaderView.swift

@@ -100,6 +100,7 @@ struct MaterialReaderView: View {
     @State private var scrollProgress: CGFloat = 0
     @State private var hasRestoredPosition = false
     @State private var restoreBlockId: String?
+    @State private var actualContentHeight: CGFloat = 1
 
     private let title: String
     private let knowledgeBaseId: String?
@@ -233,15 +234,17 @@ struct MaterialReaderView: View {
                 }
                 .padding(.horizontal, 20).padding(.top, 8).padding(.bottom, 100)
                 .background(GeometryReader { geo in
-                    Color.clear.preference(
-                        key: ScrollOffsetKey.self,
-                        value: geo.frame(in: .named("scroll")).minY
-                    )
+                    Color.clear
+                        .preference(key: ScrollOffsetKey.self, value: geo.frame(in: .named("scroll")).minY)
+                        .onAppear { actualContentHeight = geo.size.height }
                 })
+                .onChange(of: vm.blocks.count) { _,_ in
+                    // Height will be updated by GeometryReader on next render
+                }
             }
             .coordinateSpace(name: "scroll")
             .onPreferenceChange(ScrollOffsetKey.self) { offset in
-                scrollProgress = min(1, max(0, -offset / max(contentHeightEstimate, 1)))
+                scrollProgress = min(1, max(0, -offset / max(actualContentHeight, 1)))
                 reportScrollPosition()
             }
             .onChange(of: restoreBlockId) { _, target in
@@ -256,12 +259,6 @@ struct MaterialReaderView: View {
         }
     }
 
-    /// Estimate total block area height for scroll progress calculation
-    private var contentHeightEstimate: CGFloat {
-        let count = max(vm.blocks.count, 1)
-        return CGFloat(count) * 80
-    }
-
     /// Build a NoteAnchor from the current scroll position (for quick note).
     private func buildAnchor() -> NoteAnchor? {
         guard let pos = collector.lastPosition else { return nil }