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/editor/input/pen_config.dart

@@ -4,6 +4,8 @@ import 'dart:math';
 import 'package:flutter/foundation.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
+import '../engine/stroke_geometry.dart' show kDefaultPenThinning;
+
 /// Action that can be triggered by a hardware pen button or the eraser end.
 enum PenButtonAction {
   none,
@@ -25,10 +27,13 @@ class PenConfig {
     this.fingerDrawing = false,
     this.penWidth = 0.004,
     this.highlighterWidth = 0.02,
+    this.pressureSensitivity = kDefaultPenThinning,
   })  : assert(pressureGamma >= 0.3 && pressureGamma <= 3.0,
             'pressureGamma must be in [0.3, 3.0]'),
         assert(palmRejectionMs >= 0.0 && palmRejectionMs <= 500.0,
-            'palmRejectionMs must be in [0, 500]');
+            'palmRejectionMs must be in [0, 500]'),
+        assert(pressureSensitivity >= 0.0 && pressureSensitivity <= 1.0,
+            'pressureSensitivity must be in [0, 1]');
 
   /// Which action fires when the side barrel button is held.
   final PenButtonAction sideButton;
@@ -54,6 +59,13 @@ class PenConfig {
   /// Highlighter stroke width as a fraction of the canvas width.
   final double highlighterWidth;
 
+  /// How strongly stylus pressure modulates stroke width — maps directly to
+  /// perfect_freehand's `thinning`. Range [0,1]; `0` = constant width,
+  /// higher = pressure sweeps width more (Saber's `StrokeOptions.thinning`
+  /// model). Default [kDefaultPenThinning] so the out-of-box feel and the
+  /// export golden are unchanged.
+  final double pressureSensitivity;
+
   PenConfig copyWith({
     PenButtonAction? sideButton,
     PenButtonAction? eraserEnd,
@@ -62,6 +74,7 @@ class PenConfig {
     bool? fingerDrawing,
     double? penWidth,
     double? highlighterWidth,
+    double? pressureSensitivity,
   }) {
     return PenConfig(
       sideButton: sideButton ?? this.sideButton,
@@ -71,6 +84,7 @@ class PenConfig {
       fingerDrawing: fingerDrawing ?? this.fingerDrawing,
       penWidth: penWidth ?? this.penWidth,
       highlighterWidth: highlighterWidth ?? this.highlighterWidth,
+      pressureSensitivity: pressureSensitivity ?? this.pressureSensitivity,
     );
   }
 
@@ -82,6 +96,7 @@ class PenConfig {
         'fingerDrawing': fingerDrawing,
         'penWidth': penWidth,
         'highlighterWidth': highlighterWidth,
+        'pressureSensitivity': pressureSensitivity,
       };
 
   factory PenConfig.fromJson(Map<String, dynamic> json) {
@@ -97,6 +112,8 @@ class PenConfig {
       fingerDrawing: json['fingerDrawing'] as bool? ?? false,
       penWidth: (json['penWidth'] as num?)?.toDouble() ?? 0.004,
       highlighterWidth: (json['highlighterWidth'] as num?)?.toDouble() ?? 0.02,
+      pressureSensitivity:
+          (json['pressureSensitivity'] as num?)?.toDouble() ?? kDefaultPenThinning,
     );
   }
 
@@ -111,7 +128,8 @@ class PenConfig {
           palmRejectionMs == other.palmRejectionMs &&
           fingerDrawing == other.fingerDrawing &&
           penWidth == other.penWidth &&
-          highlighterWidth == other.highlighterWidth;
+          highlighterWidth == other.highlighterWidth &&
+          pressureSensitivity == other.pressureSensitivity;
 
   @override
   int get hashCode => Object.hash(
@@ -122,6 +140,7 @@ class PenConfig {
         fingerDrawing,
         penWidth,
         highlighterWidth,
+        pressureSensitivity,
       );
 }
 
@@ -215,4 +234,11 @@ class PenConfigController extends ChangeNotifier {
     notifyListeners();
     await _persist();
   }
+
+  /// Sets [PenConfig.pressureSensitivity]. Clamped to [0, 1].
+  Future<void> setPressureSensitivity(double sensitivity) async {
+    _value = _value.copyWith(pressureSensitivity: sensitivity.clamp(0.0, 1.0));
+    notifyListeners();
+    await _persist();
+  }
 }