[#3516] feat(C#): add C# solution

problems/3516-find-closest-person/csharp/Program.cs

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+// LeetCode 3516: Find Closest Person
+// 難度: Easy
+// 日期: 2025-09-04
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+public class Solution {
+    public int FindClosest(int x, int y, int z)
+    {
+        var distanceX = Math.Abs(x - z);
+        var distanceY = Math.Abs(y - z);
+
+        return distanceX == distanceY ? 0 : distanceX > distanceY ? 2 : 1;
+    }
+}
+
+// 測試程式
+public class Program
+{
+    public static void Main()
+    {
+        TestCase1();
+        TestCase2();
+        TestCase3();
+    }
+
+    // 基本測試案例 - Example 1
+    static void TestCase1()
+    {
+        var solution = new Solution();
+        // Input: x = 2, y = 7, z = 4
+        // Expected: 1 (Person 1 距離較近: |2-4| = 2 < |7-4| = 3)
+        var actual = solution.FindClosest(2, 7, 4);
+        var expected = 1;
+        Console.WriteLine($"Test 1: Input(2, 7, 4) Expected: {expected}, Actual: {actual}, Result: {(actual == expected ? "PASS" : "FAIL")}");
+    }
+
+    // 基本測試案例 - Example 2
+    static void TestCase2()
+    {
+        var solution = new Solution();
+        // Input: x = 2, y = 5, z = 6
+        // Expected: 2 (Person 2 距離較近: |5-6| = 1 < |2-6| = 4)
+        var actual = solution.FindClosest(2, 5, 6);
+        var expected = 2;
+        Console.WriteLine($"Test 2: Input(2, 5, 6) Expected: {expected}, Actual: {actual}, Result: {(actual == expected ? "PASS" : "FAIL")}");
+    }
+
+    // 基本測試案例 - Example 3
+    static void TestCase3() {
+        var solution = new Solution();
+        // Input: x = 1, y = 5, z = 3
+        // Expected: 0 (距離相等: |1-3| = 2 = |5-3| = 2)
+        var actual = solution.FindClosest(1, 5, 3);
+        var expected = 0;
+        Console.WriteLine($"Test 3: Input(1, 5, 3) Expected: {expected}, Actual: {actual}, Result: {(actual == expected ? "PASS" : "FAIL")}");
+    }
+}

problems/3516-find-closest-person/csharp/csharp.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>