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_geometry.dart

@@ -25,12 +25,11 @@ const double kDefaultPenThinning = 0.85;
 /// perfect_freehand input-smoothing parameters, shared (single source of truth)
 /// by the on-screen painter and the export path so the two can never diverge
 /// (guarded by the screen==export parity test). [kPenStreamline] lowers the
-/// per-point lag from freehand's 0.5 default to 0.32: at 0.5 a quick flick lags
-/// so far behind the pen that a short fast stroke collapsed toward its start and
-/// rendered as a dot ("写字识别成单击") and the pen felt sluggish; 0.32 tracks the
-/// real path closely (crisper, lower-latency feel) while still damping digitizer
-/// jitter. [kPenSmoothing] keeps freehand's 0.5 corner rounding.
-const double kPenStreamline = 0.32;
+/// per-point lag from freehand's 0.5 default to 0.28: paired with
+/// [StrokePredictor] lookahead this tracks the Surface Pen more tightly while
+/// still damping digitizer jitter. [kPenSmoothing] keeps freehand's 0.5 corner
+/// rounding.
+const double kPenStreamline = 0.28;
 const double kPenSmoothing = 0.5;
 
 /// THE single perfect_freehand outline recipe — the raw outline points for a