Rewrite Triangulation<T> storage: raw arrays + ReadOnlyMemory, stackalloc, ref mutations (#9)

* Rewrite Triangulation memory layout: arrays + ReadOnlyMemory, stackalloc, ref mutations

Author: Copilot

src/CDT.Core/CdtUtils.cs

@@ -143,12 +143,12 @@ public static void RemapEdges(List<Edge> edges, IReadOnlyList<int> mapping)
     // -------------------------------------------------------------------------
 
     /// <summary>Extracts all unique edges from a triangle list.</summary>
-    /// <param name="triangles">The triangle list to extract edges from.</param>
+    /// <param name="triangles">The triangles to extract edges from.</param>
     /// <returns>A set containing all unique edges in the triangulation.</returns>
-    public static HashSet<Edge> ExtractEdgesFromTriangles(IReadOnlyList<Triangle> triangles)
+    public static HashSet<Edge> ExtractEdgesFromTriangles(ReadOnlySpan<Triangle> triangles)
     {
-        var edges = new HashSet<Edge>(triangles.Count * 3);
-        foreach (var t in triangles)
+        var edges = new HashSet<Edge>(triangles.Length * 3);
+        foreach (ref readonly var t in triangles)
         {
             edges.Add(new Edge(t.V0, t.V1));
             edges.Add(new Edge(t.V1, t.V2));
@@ -167,12 +167,12 @@ public static HashSet<Edge> ExtractEdgesFromTriangles(IReadOnlyList<Triangle> tr
     /// A read-only list of read-only lists: for each vertex index, the list of adjacent triangle indices.
     /// </returns>
     public static IReadOnlyList<IReadOnlyList<int>> CalculateTrianglesByVertex(
-        IReadOnlyList<Triangle> triangles,
+        ReadOnlySpan<Triangle> triangles,
         int verticesCount)
     {
         var result = new List<int>[verticesCount];
         for (int i = 0; i < verticesCount; i++) result[i] = new List<int>();
-        for (int i = 0; i < triangles.Count; i++)
+        for (int i = 0; i < triangles.Length; i++)
         {
             var t = triangles[i];
             result[t.V0].Add(i);

src/CDT.Core/TopologyVerifier.cs

@@ -19,16 +19,16 @@ public static class TopologyVerifier
     public static bool VerifyTopology<T>(Triangulation<T> cdt)
         where T : unmanaged, IFloatingPoint<T>, IMinMaxValue<T>, IRootFunctions<T>
     {
-        var triangles = cdt.Triangles;
-        var vertices = cdt.Vertices;
+        var triangles = cdt.Triangles.Span;
+        var vertices = cdt.Vertices.Span;
 
-        for (int iT = 0; iT < triangles.Count; iT++)
+        for (int iT = 0; iT < triangles.Length; iT++)
         {
             var t = triangles[iT];
             // Verify non-invalid vertices
             if (t.V0 == Indices.NoVertex || t.V1 == Indices.NoVertex || t.V2 == Indices.NoVertex)
                 return false;
-            if (t.V0 >= vertices.Count || t.V1 >= vertices.Count || t.V2 >= vertices.Count)
+            if (t.V0 >= vertices.Length || t.V1 >= vertices.Length || t.V2 >= vertices.Length)
                 return false;
             // No degenerate (same-vertex) triangles
             if (t.V0 == t.V1 || t.V1 == t.V2 || t.V0 == t.V2)
@@ -39,7 +39,7 @@ public static bool VerifyTopology<T>(Triangulation<T> cdt)
             {
                 int iN = t.GetNeighbor(i);
                 if (iN == Indices.NoNeighbor) continue;
-                if (iN >= triangles.Count) return false;
+                if (iN >= triangles.Length) return false;
                 var tN = triangles[iN];
                 // Neighbor must reference us back
                 if (tN.N0 != iT && tN.N1 != iT && tN.N2 != iT)