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/Ui/SettingsTabs/Tabs.cs

@@ -1,12 +1,12 @@
-using ChatTwo.Code;
-using ChatTwo.Resources;
-using ChatTwo.Util;
+using HellionChat.Code;
+using HellionChat.Resources;
+using HellionChat.Util;
 using Dalamud.Interface;
 using Dalamud.Interface.Utility.Raii;
 using Dalamud.Bindings.ImGui;
 using Dalamud.Game.ClientState.Objects.SubKinds;
 
-namespace ChatTwo.Ui.SettingsTabs;
+namespace HellionChat.Ui.SettingsTabs;
 
 internal sealed class Tabs : ISettingsTab
 {