feat(safety): refuse to load when Chat 2 is also active

When the user has both Hellion Chat and upstream Chat 2 installed and
loaded, the parallel chat-window replacement and Hook collisions can
crash FFXIV at frame boundaries. The new detector inspects
IDalamudPluginInterface.InstalledPlugins and throws an
InvalidOperationException with a localized message if Chat 2 is loaded,
which Dalamud surfaces cleanly instead of letting the load proceed
into a runtime crash.

Bilingual messages (EN/DE) follow the existing HellionStrings pattern.

ChatTwo/ChatTwoConflictDetector.cs

@@ -0,0 +1,27 @@
+using System.Linq;
+using ChatTwo.Resources;
+using Dalamud.Plugin;
+
+namespace ChatTwo;
+
+internal static class ChatTwoConflictDetector
+{
+    private const string UpstreamInternalName = "ChatTwo";
+
+    public static void ThrowIfChatTwoIsLoaded(IDalamudPluginInterface pluginInterface)
+    {
+        var conflict = pluginInterface.InstalledPlugins
+            .FirstOrDefault(p =>
+                p.InternalName == UpstreamInternalName &&
+                p.IsLoaded);
+
+        if (conflict is null)
+            return;
+
+        var message = HellionStrings.ChatTwoConflictTitle + "\n\n" +
+                      HellionStrings.ChatTwoConflictBody + "\n\n" +
+                      HellionStrings.ChatTwoConflictAction;
+
+        throw new System.InvalidOperationException(message);
+    }
+}

ChatTwo/Plugin.cs

@@ -94,6 +94,12 @@ public sealed class Plugin : IDalamudPlugin
 
     public Plugin()
     {
+        // Refuse to start if upstream Chat 2 is loaded — prevents IPC
+        // channel collisions and double-replacement of the in-game chat
+        // window. Throwing here makes Dalamud abort the load cleanly with
+        // our localized message instead of crashing FFXIV mid-frame.
+        ChatTwoConflictDetector.ThrowIfChatTwoIsLoaded(Interface);
+
         try
         {
             GameStarted = Process.GetCurrentProcess().StartTime.ToUniversalTime();

ChatTwo/Resources/HellionStrings.Designer.cs

@@ -270,4 +270,9 @@ internal class HellionStrings
     internal static string Hint_v061_PopOutHeader_Body => Get(nameof(Hint_v061_PopOutHeader_Body));
     internal static string Hint_v061_PopOutHeader_Ack => Get(nameof(Hint_v061_PopOutHeader_Ack));
     internal static string Hint_v061_PopOutHeader_OpenSettings => Get(nameof(Hint_v061_PopOutHeader_OpenSettings));
+
+    // Hellion Chat — v1.0.0 Chat 2 parallel-load conflict detection
+    internal static string ChatTwoConflictTitle => Get(nameof(ChatTwoConflictTitle));
+    internal static string ChatTwoConflictBody => Get(nameof(ChatTwoConflictBody));
+    internal static string ChatTwoConflictAction => Get(nameof(ChatTwoConflictAction));
 }

ChatTwo/Resources/HellionStrings.de.resx

@@ -609,4 +609,13 @@
   <data name="Hint_v061_PopOutHeader_OpenSettings" xml:space="preserve">
     <value>Einstellungen öffnen</value>
   </data>
+  <data name="ChatTwoConflictTitle" xml:space="preserve">
+    <value>Hellion Chat kann nicht starten, solange Chat 2 geladen ist.</value>
+  </data>
+  <data name="ChatTwoConflictBody" xml:space="preserve">
+    <value>Hellion Chat ist ein eigenständiger Fork von Chat 2. Beide Plugins ersetzen dasselbe Chat-Fenster im Spiel und würden zur Laufzeit kollidieren.</value>
+  </data>
+  <data name="ChatTwoConflictAction" xml:space="preserve">
+    <value>Chat 2 in /xlplugins deaktivieren, danach Hellion Chat erneut aktivieren.</value>
+  </data>
 </root>

ChatTwo/Resources/HellionStrings.resx

@@ -609,4 +609,13 @@
   <data name="Hint_v061_PopOutHeader_OpenSettings" xml:space="preserve">
     <value>Open Settings</value>
   </data>
+  <data name="ChatTwoConflictTitle" xml:space="preserve">
+    <value>Hellion Chat cannot start while Chat 2 is loaded.</value>
+  </data>
+  <data name="ChatTwoConflictBody" xml:space="preserve">
+    <value>Hellion Chat is a standalone fork of Chat 2. Both plugins replace the same in-game chat window and would conflict at runtime if loaded together.</value>
+  </data>
+  <data name="ChatTwoConflictAction" xml:space="preserve">
+    <value>Disable Chat 2 in /xlplugins, then re-enable Hellion Chat.</value>
+  </data>
 </root>