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>

test/diagnostics_test.dart

@@ -0,0 +1,36 @@
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/diagnostics/badnote_log.dart';
+import 'package:badnote/diagnostics/pen_event_ring.dart';
+import 'package:badnote/diagnostics/frame_sampler.dart';
+
+void main() {
+  test('PenEventRing retains recent events', () {
+    final ring = PenEventRing.instance;
+    ring.clear();
+    ring.recordArbiter(
+      activeCount: 1,
+      deviceKind: 'stylus',
+      draw: true,
+      fingerDrawing: false,
+    );
+    expect(ring.recent(), isNotEmpty);
+    expect(ring.toJsonList().first['decision'], 'draw');
+  });
+
+  test('FrameSampler flags over-budget frames', () {
+    FrameSampler.instance.reset();
+    FrameSampler.instance.record('test', 8);
+    FrameSampler.instance.record('slow', 30);
+    expect(FrameSampler.instance.overBudget, 1);
+    expect(FrameSampler.instance.summary()['total'], 2);
+  });
+
+  test('BadNoteLog ring accepts entries without start', () {
+    BadNoteLog.instance.info(LogSubsystem.diag, 'unit_test_ping');
+    expect(
+      BadNoteLog.instance.snapshotRing().any((e) => e['msg'] == 'unit_test_ping'),
+      isTrue,
+    );
+  });
+}