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/stroke.dart

@@ -0,0 +1,10 @@
+// Canonical stroke model surface.
+//
+// Historical baggage had three parallel types (PenStroke / EditorStroke /
+// InkStroke). New code MUST import from this barrel and prefer [EditorStroke]
+// for engine/storage. UI adapters convert at the edge.
+//
+// Do not add a fourth model.
+
+export '../engine/stroke_model.dart' show EditorStroke, EditorPoint, EditorTool;
+export '../canvas/pen_stroke.dart' show PenStroke, PenPoint, PenStrokeKind;