Fixing stair-stepped brush strokes with timestamp kinematics

Fast brush flicks were starving the interpolation curve, leaving ugly jagged staircase edges on the canvas. When relying strictly on raw coordinate deltas, rapid trackpad swipes didn't provide enough data points for smooth splines. The fix was moving to a timestamp-normalized kinematic model. I implemented a dynamic Centripetal Catmull-Rom ingester that scales subdivision steps based on true elapsed millisecond velocity. Now, even a 2,500px/s flick renders with perfect tangent continuity and natural taper.