feat: add ThemeSwitchSelfTestStep + ISelfTestRegistry wiring

Registers a single SelfTestStep that exercises Plugin.ThemeRegistry.Switch
through the live theme list. Verified in-game via /xldev SelfTest tab on
2026-05-08; Plugin loads cleanly with the RegisterTestSteps call and the
step runs the theme cycle as expected.

Folder is HellionChat/SelfTest/ (singular). Future steps may rename to
SelfTests/ to match the local Plan v4 convention.

HellionChat/Plugin.cs

@@ -41,6 +41,7 @@ public sealed class Plugin : IDalamudPlugin
     [PluginService] public static IAddonLifecycle AddonLifecycle { get; private set; } = null!;
     [PluginService] public static IPlayerState PlayerState { get; private set; } = null!;
     [PluginService] public static ISeStringEvaluator Evaluator { get; private set; } = null!;
+    [PluginService] public static ISelfTestRegistry SelfTestRegistry { get; private set; } = null!;
 
     public static Configuration Config = null!;
     public static FileDialogManager FileDialogManager { get; private set; } = null!;
@@ -452,6 +453,12 @@ public sealed class Plugin : IDalamudPlugin
             ThemeRegistry = new Themes.ThemeRegistry(customThemesDir);
             ThemeRegistry.Switch(Config.Theme);
 
+            // SelfTest hooks live alongside the live registry — the steps
+            // poll Active per frame and need the registry already wired.
+            SelfTestRegistry.RegisterTestSteps([
+                new SelfTest.ThemeSwitchSelfTestStep(this),
+            ]);
+
             // Plugin integrations register their IPC subscribers up-front so
             // Ready/Disposing events from the target plugins are caught from
             // the very first frame, even if the user's Honorific reloads