feat(pen): pressure-responsive width, configurable thinning, native Windows pen (tilt/buttons)

W1 — Custom pen width + pressure sensitivity (Saber-style):
- Root cause of "压感没用": perfect_freehand 1.0.4 IGNORES real stylus pressure
  (hardcodes radius=size/2 when simulatePressure=false) — width never tracked pen
  force. Upgraded perfect_freehand ^1.0.0 -> ^2.0.0 (honors real pressure); migrated
  all 5 getStroke call sites to the 2.x API (PointVector / StrokeOptions / Offset).
- De-hardcoded `thinning` into `kDefaultPenThinning` (0.85), single source shared by
  the on-screen painter and the PDF export path; exposed as PenConfig.pressureSensitivity
  with a Pressure Sensitivity slider; live-applies via a config listener.

W3 — Native Windows pen plugin (tilt + barrel/eraser buttons):
- windows/runner/pen_channel.{h,cpp}: observe WM_POINTER at the TOP of MessageHandler
  (before HandleTopLevelWindowProc, which Flutter uses to consume pen events), read
  GetPointerPenInfo penFlags + tilt, stream over EventChannel('badnote/pen'); non-consuming.
- PenInputService: single latched hardware state (no Win32-pointerId<->event.pointer
  correlation); graceful no-op off-Windows.
- pen_canvas maps barrel/inverted/eraser through PenConfig.sideButton/eraserEnd
  (eraser/undo/toggleTool/pan) and captures tilt into PenPoint.tilt -> EditorPoint.tilt.

W2 — Zoom flicker: page raster isolated in its own RepaintBoundary (safe interim);
definitive crisp-on-zoom fix gated on the on-device root-cause probe (plan M3).

Plans: ralplan-consensus plan at docs/plans/2026-06-22-badnote-pen-polish.md
(Architect APPROVE-WITH-MUST-FIX M1-M4 + Critic ITERATE->APPROVE).

Tests: 58/58 pass incl. shared-thinning invariant + thinning-affects-outline +
tilt-adapter round-trip. flutter analyze clean; linux debug build OK.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/widgets/ink_canvas.dart

@@ -528,7 +528,7 @@ class _InkPainter extends CustomPainter {
   ) {
     final pfPoints = points
         .map(
-          (p) => pf.Point(
+          (p) => pf.PointVector(
             p.x,
             p.y,
             pressureCurve.apply(p.pressure).clamp(0.0, 1.0),
@@ -542,25 +542,25 @@ class _InkPainter extends CustomPainter {
 
     final outlinePoints = pf.getStroke(
       pfPoints,
-      size: strokeWidth,
-      thinning: thinning,
-      smoothing: 0.5,
-      streamline: 0.5,
-      taperStart: 0.0,
-      taperEnd: 0.0,
-      capStart: true,
-      capEnd: true,
-      simulatePressure: tool != PenTool.marker && tool != PenTool.highlighter,
-      isComplete: isComplete,
+      options: pf.StrokeOptions(
+        size: strokeWidth,
+        thinning: thinning,
+        smoothing: 0.5,
+        streamline: 0.5,
+        // 2.x defaults: no taper + capped ends (was taperStart/End:0 +
+        // capStart/End:true in 1.0.4).
+        simulatePressure: tool != PenTool.marker && tool != PenTool.highlighter,
+        isComplete: isComplete,
+      ),
     );
 
     if (outlinePoints.isEmpty) return;
 
     final path = Path();
-    path.moveTo(outlinePoints[0].x, outlinePoints[0].y);
+    path.moveTo(outlinePoints[0].dx, outlinePoints[0].dy);
 
     for (int i = 1; i < outlinePoints.length; i++) {
-      path.lineTo(outlinePoints[i].x, outlinePoints[i].y);
+      path.lineTo(outlinePoints[i].dx, outlinePoints[i].dy);
     }
     path.close();