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>

windows/runner/flutter_window.cpp

@@ -4,6 +4,7 @@
 
 #include "flutter/generated_plugin_registrant.h"
 #include "ocr_channel.h"
+#include "pen_channel.h"
 
 FlutterWindow::FlutterWindow(const flutter::DartProject& project)
     : project_(project) {}
@@ -27,6 +28,7 @@ bool FlutterWindow::OnCreate() {
   }
   RegisterPlugins(flutter_controller_->engine());
   RegisterOcrChannel(flutter_controller_->engine());
+  RegisterPenChannel(flutter_controller_->engine());
   SetChildContent(flutter_controller_->view()->GetNativeWindow());
 
   flutter_controller_->engine()->SetNextFrameCallback([&]() {
@@ -53,6 +55,9 @@ LRESULT
 FlutterWindow::MessageHandler(HWND hwnd, UINT const message,
                               WPARAM const wparam,
                               LPARAM const lparam) noexcept {
+  // Observe pen messages before Flutter consumes WM_POINTER events.
+  ObservePenMessage(message, wparam, lparam);
+
   // Give Flutter, including plugins, an opportunity to handle window messages.
   if (flutter_controller_) {
     std::optional<LRESULT> result =