Rewrite Triangulation<T> storage: raw arrays + ReadOnlyMemory, stackalloc, ref mutations

[Copilot]

Replace the three List<T> backing stores with raw arrays + explicit count fields, exposing contiguous-memory read-only views and eliminating unnecessary copies throughout the hot paths.

Storage & public API

• List<V2d<T>> → V2d<T>[] + int _verticesCount; Vertices now returns ReadOnlyMemory<V2d<T>>
• List<Triangle> → Triangle[] + int _trianglesCount; Triangles now returns ReadOnlyMemory<Triangle>
• List<int> _vertTris → int[] + int _vertTrisCount
• Pre-allocation via ArrayEnsureCapacity on first insertion; doubling growth via ArrayAdd

// Before
public IReadOnlyList<V2d<T>> Vertices => _vertices;
public IReadOnlyList<Triangle> Triangles => _triangles;

// After
public ReadOnlyMemory<V2d<T>> Vertices => new(_vertices, 0, _verticesCount);
public ReadOnlyMemory<Triangle> Triangles => new(_triangles, 0, _trianglesCount);

stackalloc

InsertVertices_Randomized and InsertVertices_KDTreeBFS use stackalloc int[n] when n ≤ StackAllocThreshold (512), falling back to heap otherwise. NthElement changed from int[] to Span<int>.

ref mutations — eliminate copy-back pattern

All read-modify-write triangle mutations converted from the var t = …; mutate; _triangles[i] = t pattern to ref var t = ref _triangles[i]:

• Both ChangeNeighbor overloads
• TriangulatePseudoPolygonIteration (was doing 4–6 round-trips per call)
• RemoveTriangles neighbor remapping loop
• FinalizeTriangulation vertex-offset loop

Span-based utilities

• VertexXComparer/VertexYComparer take V2d<T>[] directly — removes IReadOnlyList<T> interface dispatch on every compare
• Box2d<T>.Envelop(ReadOnlySpan<V2d<T>>) overload added; used in InitKdTree
• CdtUtils.ExtractEdgesFromTriangles and CalculateTrianglesByVertex accept ReadOnlySpan<Triangle> with ref readonly inner loops
• TopologyVerifier works off .Span from both memory properties

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