refactor(namespace): rename ChatTwo.* to HellionChat.* across all source files

81 namespace declarations and 100 using directives converted via sed,
plus two FQN-aliases (ChatTwoPartyFinderPayload in PayloadHandler.cs and
ModifierFlag in KeybindManager.cs) updated. Critical: Language.Designer.cs
and HellionStrings.Designer.cs ResourceManager string arguments updated
synchronously — these are runtime reflection lookups not caught by the
C# compiler.

Two intentional ChatTwo references remain: the legacy migration path
'ChatTwo.json' in Plugin.cs (still points to upstream Chat 2's config
file by design) and the InternalsVisibleTo declaration in
AssemblyInfo.cs (handled in the upcoming repo-folder rename task).

The local alias names 'ChatTwoPartyFinderPayload' and 'ChatTwoConflictDetector'
are preserved as local symbols; only their target namespaces and references
changed.

ChatTwo/MessageManager.cs

@@ -1,9 +1,9 @@
 using System.Collections.Concurrent;
 using System.Diagnostics;
 using System.Text;
-using ChatTwo.Code;
-using ChatTwo.Resources;
-using ChatTwo.Util;
+using HellionChat.Code;
+using HellionChat.Resources;
+using HellionChat.Util;
 using Dalamud.Game.Chat;
 using Dalamud.Game.Text;
 using Dalamud.Game.Text.SeStringHandling;
@@ -15,7 +15,7 @@ using Lumina.Text.Expressions;
 using Lumina.Text.Payloads;
 using Lumina.Text.ReadOnly;
 
-namespace ChatTwo;
+namespace HellionChat;
 
 internal class MessageManager : IAsyncDisposable
 {