✨ 添加C#实现的API签名工具，包括核心库、命令行工具和使用文档，支持多种签名算法和环境变量配置。

csharp/src/ApiSigner/SignatureAlgorithm.cs

@@ -0,0 +1,76 @@
+using System;
+
+namespace SoundForce.ApiSigner
+{
+    /// <summary>
+    /// 签名算法类型
+    /// </summary>
+    public enum SignatureAlgorithm
+    {
+        /// <summary>
+        /// MD5算法（默认、最快）
+        /// </summary>
+        Md5,
+
+        /// <summary>
+        /// SHA1算法
+        /// </summary>
+        Sha1,
+
+        /// <summary>
+        /// SHA256算法
+        /// </summary>
+        Sha256,
+
+        /// <summary>
+        /// HMAC-SHA256算法（最安全）
+        /// </summary>
+        HmacSha256
+    }
+
+    /// <summary>
+    /// 签名算法扩展方法
+    /// </summary>
+    public static class SignatureAlgorithmExtensions
+    {
+        /// <summary>
+        /// 从字符串解析算法类型
+        /// </summary>
+        /// <param name="algorithm">算法字符串</param>
+        /// <returns>签名算法枚举</returns>
+        /// <exception cref="ArgumentException">如果算法无效</exception>
+        public static SignatureAlgorithm FromString(string? algorithm)
+        {
+            if (string.IsNullOrEmpty(algorithm))
+                return SignatureAlgorithm.Md5;
+
+            return algorithm.ToUpperInvariant() switch
+            {
+                "MD5" => SignatureAlgorithm.Md5,
+                "SHA1" => SignatureAlgorithm.Sha1,
+                "SHA256" => SignatureAlgorithm.Sha256,
+                "HMAC_SHA256" => SignatureAlgorithm.HmacSha256,
+                "HMACSHA256" => SignatureAlgorithm.HmacSha256,
+                "HMAC-SHA256" => SignatureAlgorithm.HmacSha256,
+                _ => throw new ArgumentException($"无效的签名算法: {algorithm}", nameof(algorithm))
+            };
+        }
+
+        /// <summary>
+        /// 获取算法的字符串表示
+        /// </summary>
+        /// <param name="algorithm">签名算法</param>
+        /// <returns>算法的字符串表示</returns>
+        public static string ToString(this SignatureAlgorithm algorithm)
+        {
+            return algorithm switch
+            {
+                SignatureAlgorithm.Md5 => "MD5",
+                SignatureAlgorithm.Sha1 => "SHA1",
+                SignatureAlgorithm.Sha256 => "SHA256",
+                SignatureAlgorithm.HmacSha256 => "HMAC-SHA256",
+                _ => "UNKNOWN"
+            };
+        }
+    }
+}