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/engine/stroke_predictor.dart

@@ -0,0 +1,56 @@
+// Lightweight stroke prediction — extrapolates the next point from recent
+// velocity so the live stroke tip leads the digitizer slightly (lower perceived
+// latency). Not a full ink-stroke-modeler; intentionally small and testable.
+
+import 'dart:ui';
+
+class PredictedPoint {
+  const PredictedPoint(this.offset, this.pressure);
+  final Offset offset;
+  final double pressure;
+}
+
+class StrokePredictor {
+  StrokePredictor({this.lookaheadMs = 12});
+
+  /// How far ahead to project, in milliseconds of recent velocity.
+  final double lookaheadMs;
+
+  Offset? _prev;
+  double? _prevPressure;
+  DateTime? _prevAt;
+  Offset _velocity = Offset.zero;
+
+  void reset() {
+    _prev = null;
+    _prevPressure = null;
+    _prevAt = null;
+    _velocity = Offset.zero;
+  }
+
+  /// Feed a real sample; returns an optional predicted tip ahead of [point].
+  PredictedPoint? observe(Offset point, double pressure, {DateTime? at}) {
+    final now = at ?? DateTime.now();
+    if (_prev != null && _prevAt != null) {
+      final dtMs = now.difference(_prevAt!).inMicroseconds / 1000.0;
+      if (dtMs > 0.5 && dtMs < 80) {
+        final raw = (point - _prev!) * (1000.0 / dtMs);
+        // EMA blend to avoid jerky predictions.
+        _velocity = Offset(
+          _velocity.dx * 0.55 + raw.dx * 0.45,
+          _velocity.dy * 0.55 + raw.dy * 0.45,
+        );
+      }
+    }
+    _prev = point;
+    _prevPressure = pressure;
+    _prevAt = now;
+
+    if (_velocity.distance < 40) return null; // idle / slow — no predict
+    final tip = point + _velocity * (lookaheadMs / 1000.0);
+    return PredictedPoint(tip, pressure);
+  }
+
+  /// Last known pressure (for predicted tip).
+  double get lastPressure => _prevPressure ?? 0.5;
+}