feat(f5): configurable pen pressure curve (floor + gamma) pure core

The user's repeated "可配置笔" ask, as a pure value type: raw normalized
pressure is pre-shaped into [floor, 1] via a min-width floor (the plan's
marker fixed-pressure floor) and a gamma response (γ<1 = more sensitive at light
touch, γ>1 = firmer). Clamps out-of-range + NaN inputs; endpoints anchored at
floor and 1. Widget-free/storage-free; PenConfig + the canvas wire it later
(live-path, on-device validated).

flutter analyze lib/editor clean; 145/145 tests (+6).

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

test/pressure_curve_test.dart

@@ -0,0 +1,49 @@
+// Tests for the configurable pen-pressure response (F5).
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/input/pressure_curve.dart';
+
+void main() {
+  test('linear is the identity over [0,1]', () {
+    const c = PressureCurve.linear;
+    expect(c.apply(0.0), closeTo(0.0, 1e-9));
+    expect(c.apply(0.5), closeTo(0.5, 1e-9));
+    expect(c.apply(1.0), closeTo(1.0, 1e-9));
+  });
+
+  test('floor raises the minimum, endpoints stay anchored at floor and 1', () {
+    const c = PressureCurve(floor: 0.4);
+    expect(c.apply(0.0), closeTo(0.4, 1e-9)); // floor
+    expect(c.apply(1.0), closeTo(1.0, 1e-9)); // full
+    expect(c.apply(0.5), closeTo(0.4 + 0.6 * 0.5, 1e-9)); // 0.7
+  });
+
+  test('gamma < 1 boosts light pressure (more sensitive)', () {
+    const c = PressureCurve(gamma: 0.5);
+    // pow(0.25, 0.5) = 0.5 → output 0.5 > input 0.25
+    expect(c.apply(0.25), closeTo(0.5, 1e-9));
+    expect(c.apply(0.25), greaterThan(0.25));
+  });
+
+  test('gamma > 1 dampens light pressure (firmer)', () {
+    const c = PressureCurve(gamma: 2.0);
+    expect(c.apply(0.5), closeTo(0.25, 1e-9)); // 0.5^2
+    expect(c.apply(0.5), lessThan(0.5));
+  });
+
+  test('output is clamped: out-of-range and NaN inputs are safe', () {
+    const c = PressureCurve(floor: 0.2);
+    expect(c.apply(-1.0), closeTo(0.2, 1e-9)); // clamped to 0 → floor
+    expect(c.apply(2.0), closeTo(1.0, 1e-9)); // clamped to 1 → full
+    expect(c.apply(double.nan), closeTo(0.2, 1e-9)); // NaN → 0 → floor
+  });
+
+  test('marker-style high floor compresses the range toward full width', () {
+    const marker = PressureCurve(floor: 0.9);
+    expect(marker.apply(0.0), closeTo(0.9, 1e-9));
+    expect(marker.apply(1.0), closeTo(1.0, 1e-9));
+    // Even a light touch stays near full width.
+    expect(marker.apply(0.1), greaterThan(0.9));
+  });
+}