feat: unified shell, diagnostics pack, native Office, sticky board

Make Surface remote debugging and classroom workflows viable: always-on
structured logs with one-click zip export, a single AppShell chrome,
OOXML PPTX/DOCX annotation without LibreOffice, and a first-class sticky
board. Also drop spike/legacy ink widgets and tighten pen feel
(predictor, PenInfoHistory, page-tile layer).

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/editor/pdf/page_tile_layer.dart

@@ -0,0 +1,61 @@
+// Double-buffer helper on top of [PageTileCache] to kill zoom white-flash:
+// keep painting the last good tile while a higher-DPI raster is in flight.
+
+import 'dart:ui' as ui;
+
+import 'package:flutter/widgets.dart';
+
+import 'page_tile_cache.dart';
+
+/// Holds the "last good" page image for the currently visible page so a zoom
+/// settle never exposes an empty frame (plan W2 / R11).
+class PageTileLayer extends ChangeNotifier {
+  PageTileLayer({PageTileCache? cache}) : _cache = cache ?? PageTileCache();
+
+  final PageTileCache _cache;
+  ui.Image? _lastGood;
+  TileKey? _lastKey;
+
+  PageTileCache get cache => _cache;
+  ui.Image? get lastGood => _lastGood;
+  TileKey? get lastKey => _lastKey;
+
+  /// Snap continuous zoom to a coarse DPI bucket (avoids a tile per frame).
+  static int dpiBucketFor(double zoom, {double baseDpi = 96, double step = 0.5}) {
+    final raw = zoom / step;
+    final snapped = raw.round().clamp(1, 16);
+    return (snapped * step * baseDpi).round();
+  }
+
+  /// Promote [image] as the last-good tile for [key].
+  void put(TileKey key, ui.Image image) {
+    _cache.put(key, image);
+    _lastGood = image;
+    _lastKey = key;
+    notifyListeners();
+  }
+
+  /// Prefer exact bucket; else fall back to last-good so zoom never blanks.
+  ui.Image? resolve(TileKey key) {
+    final hit = _cache.get(key);
+    if (hit != null) {
+      _lastGood = hit;
+      _lastKey = key;
+      return hit;
+    }
+    return _lastGood;
+  }
+
+  void clear() {
+    _lastGood = null;
+    _lastKey = null;
+    _cache.dispose();
+    notifyListeners();
+  }
+
+  @override
+  void dispose() {
+    clear();
+    super.dispose();
+  }
+}