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/canvas/pen_editor_screen.dart

@@ -9,9 +9,11 @@ import 'package:flutter/material.dart';
 import 'package:pdfrx/pdfrx.dart';
 
 import '../../services/database_service.dart';
+import '../engine/stroke_geometry.dart' show kDefaultPenThinning;
 import '../engine/stroke_model.dart';
 import '../engine/undo_stack.dart';
 import '../input/pen_config.dart';
+import '../input/pen_input_service.dart';
 import '../persistence/editor_repository.dart';
 import '../persistence/save_scheduler.dart';
 import '../ui/pen_settings_page.dart';
@@ -98,8 +100,8 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
   Color _color = Colors.black;
   bool _allowFingerDrawing = false;
 
-  /// Pen width as a fraction of page width.
-  static const double _penWidthFraction = 0.004;
+  /// Pen width as a fraction of page width (base; pressure thins it down).
+  static const double _penWidthFraction = 0.006;
   static const double _highlighterWidthFraction = 0.02;
 
   static const List<Color> _palette = [
@@ -114,6 +116,9 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
   void initState() {
     super.initState();
     _documentId = _documentIdFromPath(widget.pdfPath);
+    // Begin listening to the native Windows pen plugin (barrel/eraser/tilt).
+    // No-op on platforms without the plugin (W3).
+    PenInputService.instance.start();
     _initPersistence();
     _initPenConfig();
     _open();
@@ -125,6 +130,9 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
       controller.dispose();
       return;
     }
+    // Rebuild the editor when pen settings change (width, pressure
+    // sensitivity, button mappings) so the live canvas reflects them.
+    controller.addListener(_onPenConfigChanged);
     setState(() {
       _penConfig = controller;
       // Adopt the persisted finger-drawing preference as the initial local
@@ -134,6 +142,10 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     });
   }
 
+  void _onPenConfigChanged() {
+    if (mounted) setState(() {});
+  }
+
   Future<void> _initPersistence() async {
     final service = await DatabaseService.getInstance();
     if (!mounted) return;
@@ -159,7 +171,7 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
       loaded[pageIndex] = entry.value
           .map((es) => PenStroke(
                 points: es.points
-                    .map((ep) => PenPoint(ep.x, ep.y, ep.pressure))
+                    .map((ep) => PenPoint(ep.x, ep.y, ep.pressure, tilt: ep.tilt))
                     .toList(),
                 color: es.color,
                 width: es.width,
@@ -211,6 +223,7 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     _document?.dispose();
     _transform.dispose();
     _penConfig?.dispose();
+    PenInputService.instance.stop();
     super.dispose();
   }
 
@@ -308,6 +321,33 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     _schedulePageSave(_pageIndex, List<PenStroke>.of(snapshot));
   }
 
+  /// Cycle pen → highlighter → eraser → pen (for the toggleTool button action).
+  void _cycleTool() {
+    setState(() {
+      _tool = switch (_tool) {
+        CanvasTool.pen => CanvasTool.highlighter,
+        CanvasTool.highlighter => CanvasTool.eraser,
+        CanvasTool.eraser => CanvasTool.pen,
+      };
+    });
+  }
+
+  /// Handle a hardware pen-button action delivered by [PenCanvas] (W3).
+  /// `eraser` and `pan` are handled inside the canvas; here we map the
+  /// edge-triggered ones.
+  void _handlePenButtonAction(PenButtonAction action) {
+    switch (action) {
+      case PenButtonAction.undo:
+        _performUndo();
+      case PenButtonAction.toggleTool:
+        _cycleTool();
+      case PenButtonAction.eraser:
+      case PenButtonAction.pan:
+      case PenButtonAction.none:
+        break;
+    }
+  }
+
   /// Toggle finger-drawing, keeping the local state and the persisted config
   /// (when loaded) in sync.
   void _toggleFingerDrawing() {
@@ -457,6 +497,13 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
                 ? (_penConfig?.value.highlighterWidth ??
                     _highlighterWidthFraction)
                 : (_penConfig?.value.penWidth ?? _penWidthFraction),
+            thinning:
+                _penConfig?.value.pressureSensitivity ?? kDefaultPenThinning,
+            sideButtonAction:
+                _penConfig?.value.sideButton ?? PenButtonAction.eraser,
+            eraserEndAction:
+                _penConfig?.value.eraserEnd ?? PenButtonAction.eraser,
+            onPenButtonAction: _handlePenButtonAction,
             allowFingerDrawing: _allowFingerDrawing,
             onPenDebug: _showPenDebug
                 ? (s) => setState(() => _penDebug = s)