feat: define path smoothing contracts

ClumsyPilot/ParkrobTrajplanner/PathSmoothing/Contracts/CubicBSplineOptions.cs

@@ -0,0 +1,8 @@
+namespace MultiWheelC.TrajectoryPlanning.PathSmoothing;
+
+/// <summary>三次 B 样条平滑参数。</summary>
+public sealed class CubicBSplineOptions
+{
+    /// <summary>端点切向控制柄相对于相邻弦长的比例。</summary>
+    public double EndpointTangentScale { get; set; } = 1d / 3d;
+}