[修正] EdgeLinkManager.Parse 改用 .NET Standard 2.0 相容 API

Unity 2020.3 (.NET Standard 2.0 預設) 不支援：
  - String.Split(string, StringSplitOptions) overload (.NET 5+)
  - System.Range / System.Index (.NET Standard 2.1+)
  - x[..i] / x[i..] 範圍語法

改為：
  - .Split(new[] { fieldDelimiter }, StringSplitOptions.RemoveEmptyEntries)
  - .Substring(0, i) / .Substring(i + kvSeparator.Length)

其他 Runtime 檔 (Client / TcpListener / UdpClient) 已用 Substring，OK。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: changlin.cai

SDK/Unity/Package/Runtime/EdgeLinkManager.cs

@@ -200,11 +200,11 @@ private Dictionary<string, string> Parse(string msg)
             result["raw"] = msg;
             return result;
         }
-        foreach (var part in msg.Split(fieldDelimiter, StringSplitOptions.RemoveEmptyEntries))
+        foreach (var part in msg.Split(new[] { fieldDelimiter }, StringSplitOptions.RemoveEmptyEntries))
         {
             int i = part.IndexOf(kvSeparator, StringComparison.Ordinal);
             if (i < 0) continue;
-            result[part[..i].Trim()] = part[(i + kvSeparator.Length)..].Trim();
+            result[part.Substring(0, i).Trim()] = part.Substring(i + kvSeparator.Length).Trim();
         }
         return result;
     }