docs: document smoothing processing core

ClumsyPilot/ParkrobTrajplanner/PathSmoothing/LocalG2/LocalG2CandidateBuilder.cs

@@ -18,6 +18,13 @@ internal sealed class LocalG2CandidateBuilder
     private const int MaximumSubdivisionDepth = 32;
     private static readonly double[] DerivativeScaleMultipliers = { 1d, 0.85d, 1.15d };
 
+    /// <summary>按确定的窗口与导数尺度顺序生成有限个五次 Hermite 局部 G2 候选。</summary>
+    /// <param name="originalSegment">候选所属、未经替换的单方向预处理段。</param>
+    /// <param name="region">包含曲率跳变和可用窗口变体的局部平滑区域。</param>
+    /// <param name="outputSpacingMeters">候选采样间距，单位 m，必须为正且有限。</param>
+    /// <param name="options">已校验的局部 G2 窗口、偏差和候选数限制。</param>
+    /// <param name="cancellationToken">取消令牌；取消时抛出 <see cref="OperationCanceledException"/>。</param>
+    /// <returns>只读的候选几何集合；没有可构造候选时返回空集合而非 <see langword="null"/>。</returns>
     internal IReadOnlyList<LocalG2CandidateGeometry> Build(
         PreparedDirectionSegment originalSegment,
         LocalG2SmoothingRegion region,