feat(pen): apply pressure-response curve for natural feel

"手写笔就是一个带压感的手指，没有特殊适配" — correct. The new editor fed
RAW LINEAR stylus pressure into perfect_freehand, and the pressureGamma
config (a slider in pen-settings) was read ONLY by that slider's UI and
NEVER applied to a stroke. Dead wiring, like the rest.

Wire it for real:
- PenCanvas applies PressureCurve(floor, gamma) at capture, so stored
  pressure carries the feel and live + PDF export replay identically.
- Natural defaults: gamma 0.7 (light touches register more width, rnote/
  OneNote-like) + floor 0.12 (thin strokes keep body, not scratchy).
- pen_editor threads PenConfig.pressureGamma into the canvas — the slider
  now actually changes stroke width.
- pen_config: natural default + one-time migration of the legacy inert
  gamma 1.0, guarded by a marker so a deliberate 1.0 still sticks.

Tests: pen_config_gamma_migration (4) + pressure_curve (6) pass; pen
widget regressions green. flutter analyze: 0 issues.

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

lib/editor/input/pen_config.dart

@@ -5,6 +5,7 @@ import 'package:flutter/foundation.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 import '../engine/stroke_geometry.dart' show kDefaultPenThinning;
+import 'pressure_curve.dart' show kNaturalPressureGamma;
 
 /// Action that can be triggered by a hardware pen button or the eraser end.
 enum PenButtonAction {
@@ -22,7 +23,7 @@ class PenConfig {
   const PenConfig({
     this.sideButton = PenButtonAction.eraser,
     this.eraserEnd = PenButtonAction.eraser,
-    this.pressureGamma = 1.0,
+    this.pressureGamma = kNaturalPressureGamma,
     this.palmRejectionMs = 150.0,
     this.fingerDrawing = false,
     this.penWidth = 0.004,
@@ -107,7 +108,8 @@ class PenConfig {
       eraserEnd:
           PenButtonAction.values.asNameMap()[json['eraserEnd'] as String? ?? ''] ??
               PenButtonAction.eraser,
-      pressureGamma: (json['pressureGamma'] as num?)?.toDouble() ?? 1.0,
+      pressureGamma:
+          (json['pressureGamma'] as num?)?.toDouble() ?? kNaturalPressureGamma,
       palmRejectionMs: (json['palmRejectionMs'] as num?)?.toDouble() ?? 150.0,
       fingerDrawing: json['fingerDrawing'] as bool? ?? false,
       penWidth: (json['penWidth'] as num?)?.toDouble() ?? 0.004,
@@ -161,6 +163,9 @@ class PenConfigController extends ChangeNotifier {
   /// The SharedPreferences key under which [PenConfig] JSON is stored.
   static const prefsKey = 'pen_config_v1';
 
+  /// Marker so the legacy-gamma migration in [load] runs at most once.
+  static const _gammaMigratedKey = 'pen_config_gamma_migrated_v1';
+
   final SharedPreferences _prefs;
   PenConfig _value;
 
@@ -184,6 +189,18 @@ class PenConfigController extends ChangeNotifier {
         config = const PenConfig();
       }
     }
+    // One-time migration: before this build, pressureGamma was never applied to
+    // strokes (a dead slider), so a stored 1.0 is the legacy inert default, not
+    // a deliberate "linear feel" choice. Upgrade it ONCE to the natural curve so
+    // the pen feels right out of the box. Guarded by a marker key so that, after
+    // migrating, the user is free to set gamma back to 1.0 and have it stick.
+    if (!(prefs.getBool(_gammaMigratedKey) ?? false)) {
+      if (config.pressureGamma == 1.0) {
+        config = config.copyWith(pressureGamma: kNaturalPressureGamma);
+        await prefs.setString(prefsKey, jsonEncode(config.toJson()));
+      }
+      await prefs.setBool(_gammaMigratedKey, true);
+    }
     return PenConfigController._(prefs, config);
   }
 

lib/editor/input/pressure_curve.dart

@@ -14,6 +14,15 @@
 
 import 'dart:math' as math;
 
+/// Default pressure-response exponent. <1 so light-to-medium pressure registers
+/// more width — the responsive, rnote/OneNote-like feel — instead of the raw
+/// linear mapping that made the pen feel like a pressure-sensitive finger.
+const double kNaturalPressureGamma = 0.7;
+
+/// Default minimum shaped pressure: even the lightest touch keeps ~12% of the
+/// dynamic range so thin strokes have body instead of scratchy near-zero width.
+const double kNaturalPressureFloor = 0.12;
+
 /// Maps raw normalized pressure to a shaped response in `[floor, 1]`.
 class PressureCurve {
   const PressureCurve({this.floor = 0.0, this.gamma = 1.0})