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/Util/SearchSelector.cs

@@ -4,7 +4,7 @@ using Dalamud.Bindings.ImGui;
 using System.Collections;
 using Dalamud.Interface.Utility.Raii;
 
-namespace ChatTwo.Util;
+namespace HellionChat.Util;
 
 // Modified from: https://github.com/UnknownX7/Hypostasis/blob/master/ImGui/ExcelSheet.cs
 public static class SearchSelector