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>

lib/editor/input/pressure_curve.dart

@@ -0,0 +1,40 @@
+// lib/editor/input/pressure_curve.dart
+//
+// Configurable pen-pressure response (F5 — the user's repeated "可配置笔" ask).
+// Raw normalized stylus pressure [0,1] is pre-shaped here before it reaches
+// perfect_freehand, giving two user-facing knobs:
+//   - [floor]: a minimum output (the plan's marker "min-width floor" — a fixed-
+//     pressure marker uses floor≈1.0; a pen uses 0.0).
+//   - [gamma]: the response exponent — γ<1 makes light touches register more
+//     width (more sensitive), γ>1 requires firmer pressure (less sensitive).
+//
+// Pure value type (widget-free, storage-free) so the full mapping is unit
+// tested; PenConfig / the canvas wire it later (the wiring touches the live
+// draw path and is validated on-device).
+
+import 'dart:math' as math;
+
+/// Maps raw normalized pressure to a shaped response in `[floor, 1]`.
+class PressureCurve {
+  const PressureCurve({this.floor = 0.0, this.gamma = 1.0})
+      : assert(floor >= 0.0 && floor < 1.0),
+        assert(gamma > 0.0);
+
+  /// Minimum output (>=0, <1). 0 = full dynamic range; raise toward 1 for a
+  /// fixed-pressure feel (marker).
+  final double floor;
+
+  /// Response exponent (>0). 1 = linear; <1 = more sensitive at light pressure;
+  /// >1 = firmer.
+  final double gamma;
+
+  /// Linear, full-range pen response (identity).
+  static const PressureCurve linear = PressureCurve();
+
+  /// Shape [pressure] (clamped to [0,1]) into `[floor, 1]`.
+  double apply(double pressure) {
+    final p = pressure.isNaN ? 0.0 : pressure.clamp(0.0, 1.0);
+    final shaped = gamma == 1.0 ? p : math.pow(p, gamma).toDouble();
+    return floor + (1.0 - floor) * shaped;
+  }
+}