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>

test/pen_config_gamma_migration_test.dart

@@ -0,0 +1,60 @@
+// Proves the pen-feel pressure default and its one-time migration.
+//
+// Before this build pressureGamma was a dead slider (never applied to strokes),
+// so a persisted 1.0 is the legacy inert default. load() upgrades that ONCE to
+// the natural curve so the pen feels right out of the box — but the migration
+// must not fight a user who later deliberately picks 1.0.
+
+import 'dart:convert';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+import 'package:badnote/editor/input/pen_config.dart';
+import 'package:badnote/editor/input/pressure_curve.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  test('fresh install defaults to the natural pressure curve', () async {
+    SharedPreferences.setMockInitialValues({});
+    final c = await PenConfigController.load();
+    expect(c.value.pressureGamma, kNaturalPressureGamma);
+    expect(kNaturalPressureGamma, lessThan(1.0)); // sanity: it IS a soft curve
+  });
+
+  test('legacy stored gamma 1.0 migrates to the natural curve once', () async {
+    SharedPreferences.setMockInitialValues({
+      PenConfigController.prefsKey:
+          jsonEncode(const PenConfig(pressureGamma: 1.0).toJson()),
+    });
+    final c = await PenConfigController.load();
+    expect(c.value.pressureGamma, kNaturalPressureGamma,
+        reason: 'the dead-default 1.0 should be upgraded');
+  });
+
+  test('after migrating, a deliberate gamma 1.0 sticks (no re-migration)',
+      () async {
+    // First load migrates and sets the marker.
+    SharedPreferences.setMockInitialValues({
+      PenConfigController.prefsKey:
+          jsonEncode(const PenConfig(pressureGamma: 1.0).toJson()),
+    });
+    final first = await PenConfigController.load();
+    await first.setPressureGamma(1.0); // user deliberately chooses linear
+
+    // Reload: the marker is set, so 1.0 must be respected, not re-migrated.
+    final second = await PenConfigController.load();
+    expect(second.value.pressureGamma, 1.0,
+        reason: 'migration is one-time; user choice must persist');
+  });
+
+  test('a non-default stored gamma is never touched', () async {
+    SharedPreferences.setMockInitialValues({
+      PenConfigController.prefsKey:
+          jsonEncode(const PenConfig(pressureGamma: 0.5).toJson()),
+    });
+    final c = await PenConfigController.load();
+    expect(c.value.pressureGamma, 0.5);
+  });
+}