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>

test/pen_config_test.dart

@@ -4,6 +4,8 @@
 // applyPressureCurve. No widgets, no SharedPreferences, no Flutter framework.
 
 import 'package:flutter_test/flutter_test.dart';
+import 'package:badnote/editor/engine/stroke_geometry.dart'
+    show kDefaultPenThinning;
 import 'package:badnote/editor/input/pen_config.dart';
 
 void main() {
@@ -43,9 +45,11 @@ void main() {
         fingerDrawing: true,
         penWidth: 0.008,
         highlighterWidth: 0.03,
+        pressureSensitivity: 0.42,
       );
       final restored = PenConfig.fromJson(original.toJson());
       expect(restored, equals(original));
+      expect(restored.pressureSensitivity, 0.42);
     });
 
     test('fromJson falls back to defaults for missing keys', () {
@@ -57,6 +61,9 @@ void main() {
       expect(restored.fingerDrawing, false);
       expect(restored.penWidth, 0.004);
       expect(restored.highlighterWidth, 0.02);
+      // Missing pressureSensitivity defaults to the shared thinning constant,
+      // preserving the pre-existing stroke feel + export golden (plan M4).
+      expect(restored.pressureSensitivity, kDefaultPenThinning);
     });
 
     test('fromJson falls back to defaults for unknown enum names', () {