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/canvas/pen_canvas.dart

@@ -27,6 +27,7 @@ import '../engine/stroke_model.dart';
 import '../engine/stroke_store.dart';
 import '../input/input_arbiter.dart' as arbiter;
 import '../input/pen_config.dart';
+import '../input/pressure_curve.dart';
 import '../input/pen_input_service.dart';
 import '../render/ink_picture_cache.dart';
 import '../render/live_ink_painter.dart' as render;
@@ -55,6 +56,8 @@ class PenCanvas extends StatefulWidget {
     this.maxScale = 8.0,
     this.onPenDebug,
     this.thinning = kDefaultPenThinning,
+    this.pressureGamma = kNaturalPressureGamma,
+    this.pressureFloor = kNaturalPressureFloor,
     this.sideButtonAction = PenButtonAction.eraser,
     this.eraserEndAction = PenButtonAction.eraser,
     this.onPenButtonAction,
@@ -103,6 +106,15 @@ class PenCanvas extends StatefulWidget {
   /// perfect_freehand pressure→width response, from `PenConfig.pressureSensitivity`.
   final double thinning;
 
+  /// Pressure-response exponent applied to raw stylus pressure BEFORE it reaches
+  /// perfect_freehand. <1 boosts light touches (responsive, rnote-like); 1 is
+  /// raw linear (the old "pressure-finger" feel). From `PenConfig.pressureGamma`.
+  final double pressureGamma;
+
+  /// Minimum shaped pressure, so a light stroke still has body instead of
+  /// scratchy near-zero width. From `PenConfig.pressureFloor`.
+  final double pressureFloor;
+
   /// Configured action for the pen's side barrel button (W3 — resolved against
   /// the native pen plugin's flags on Windows).
   final PenButtonAction sideButtonAction;
@@ -189,8 +201,21 @@ class _PenCanvasState extends State<PenCanvas> {
 
   /// Normalize stylus pressure to [0,1], or null when the device reports no
   /// usable pressure range (then perfect_freehand simulates pressure).
+  ///
+  /// The raw normalized force is then shaped by the pressure-response curve
+  /// (floor + gamma) so the stored pressure already carries the rnote-like feel
+  /// — and because the shaping happens at capture, the live stroke and the PDF
+  /// export replay identical pressures (no divergence).
   double? _normalizedPressure(PointerEvent event) {
     if (!_isStylus(event.kind)) return null;
+    final double? raw = _rawNormalizedPressure(event);
+    if (raw == null) return null;
+    return PressureCurve(floor: widget.pressureFloor, gamma: widget.pressureGamma)
+        .apply(raw);
+  }
+
+  /// Raw [0,1] stylus force before response shaping (see [_normalizedPressure]).
+  double? _rawNormalizedPressure(PointerEvent event) {
     final range = event.pressureMax - event.pressureMin;
     if (range > 0.0001) {
       return ((event.pressure - event.pressureMin) / range).clamp(0.0, 1.0);

lib/editor/canvas/pen_editor_screen.dart

@@ -17,6 +17,7 @@ import '../engine/undo_stack.dart';
 import '../input/diagnostic_logger.dart';
 import '../input/pen_config.dart';
 import '../input/pen_input_service.dart';
+import '../input/pressure_curve.dart' show kNaturalPressureGamma;
 import '../layout/viewport_fit.dart';
 import '../persistence/editor_repository.dart';
 import '../persistence/save_scheduler.dart';
@@ -558,6 +559,11 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
                 : (_penConfig?.value.penWidth ?? _penWidthFraction),
             thinning:
                 _penConfig?.value.pressureSensitivity ?? kDefaultPenThinning,
+            // Pressure-response shaping (the rnote-like feel). The pen-settings
+            // gamma slider now actually drives stroke width; fall back to the
+            // natural default when no config is loaded yet.
+            pressureGamma:
+                _penConfig?.value.pressureGamma ?? kNaturalPressureGamma,
             sideButtonAction:
                 _penConfig?.value.sideButton ?? PenButtonAction.eraser,
             eraserEndAction: