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>
