How to Merge and Concatenate Arrays in C#: fix two merge methods, net10.0

[vladimir-pecanac-main]

Updates the sample for the article "How to Merge and Concatenate Arrays in C#" (https://code-maze.com/csharp-merge-arrays/). Only collections-arrays/HowToMergeArrays changes.

• Both projects move from net6.0 to net10.0.
• Packages, re-queried on NuGet on 2026-10-10 (latest stable): BenchmarkDotNet 0.15.8, Microsoft.NET.Test.Sdk 18.10.1, xunit 2.9.3, xunit.runner.visualstudio 4.0.1, coverlet.collector 10.1.0. The folder keeps xunit.
• MergeUsingBlockCopy passed element counts to Buffer.BlockCopy, whose offsets and count are in bytes, so it copied a quarter of each array and left the rest zero (the .NET 10 SDK flagged both calls with CA2018). The offsets and counts are now multiplied by sizeof(int).
• MergeUsingArrayCopyWithResize resized Array.Empty<int>() instead of the first array, so the first array's values never reached the result. It now resizes a reference to firstArray and copies the second array after it; firstArray itself is left unchanged.
• The tests asserted only NotEmpty and Length, and both broken methods passed them. Every test now compares the merged contents with an expected array built without any method under test. On the old method bodies the new tests fail exactly twice (the BlockCopy and Resize tests); on the fixed bodies all 11 pass.
• The benchmark source arrays were 1,000 copies of 1 and 1,000 copies of 2, so Union returned a two-element array. They are now Enumerable.Range(0, n) and Enumerable.Range(n / 2, n), so Union returns 1.5n values and measures comparable work.
• _arraySize goes back from 50000 to 1000, which is the value the article prints before it asks the reader to change it to 50,000.
• New benchmarks MergeUsingSpanCopyTo and MergeUsingCollectionExpression (C# 12 spread), each with a test.
• The SelectMany lambda parameter animal is renamed to array.

Build: 0 warnings, 0 errors. Tests: 11 of 11 passing. dotnet list package --vulnerable --include-transitive: no vulnerable packages. SDK 10.0.302, runtime 10.0.10.