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

HellionChat/SelfTest/ThemeSwitchSelfTestStep.cs

@@ -0,0 +1,85 @@
+using Dalamud.Bindings.ImGui;
+using Dalamud.Plugin.SelfTest;
+using HellionChat.Themes;
+
+namespace HellionChat.SelfTest;
+
+// Validates the runtime theme-switch contract from the user side. The
+// caller toggles the active theme via Settings -> Theme & Layout, the
+// step polls ThemeRegistry.Active per frame and only passes once the
+// slug has moved away from the initial value and back. The ABGR cache
+// is sanity-checked on every frame: a freshly switched theme must carry
+// a populated cache, otherwise Switch() forgot the recompute and the UI
+// would still draw, just with all-transparent slots.
+internal sealed class ThemeSwitchSelfTestStep : ISelfTestStep
+{
+    private readonly Plugin plugin;
+    private string? initialSlug;
+    private bool switchedAway;
+
+    public ThemeSwitchSelfTestStep(Plugin plugin)
+    {
+        this.plugin = plugin;
+    }
+
+    public string Name => "Hellion Chat - Theme switch";
+
+    public SelfTestStepResult RunStep()
+    {
+        var registry = this.plugin.ThemeRegistry;
+        if (registry is null)
+            return SelfTestStepResult.Fail;
+
+        var active = registry.Active;
+        if (active is null)
+            return SelfTestStepResult.Fail;
+
+        if (!HasPopulatedCache(active))
+            return SelfTestStepResult.Fail;
+
+        if (this.initialSlug is null)
+        {
+            this.initialSlug = active.Slug;
+            ImGui.Text($"Initial theme: \"{this.initialSlug}\". Open Settings -> Theme & Layout and pick a different theme.");
+            return SelfTestStepResult.Waiting;
+        }
+
+        if (!this.switchedAway)
+        {
+            if (!string.Equals(active.Slug, this.initialSlug, StringComparison.OrdinalIgnoreCase))
+            {
+                this.switchedAway = true;
+                return SelfTestStepResult.Waiting;
+            }
+
+            ImGui.Text($"Switch the active theme away from \"{this.initialSlug}\".");
+            return SelfTestStepResult.Waiting;
+        }
+
+        if (!string.Equals(active.Slug, this.initialSlug, StringComparison.OrdinalIgnoreCase))
+        {
+            ImGui.Text($"Switch back to \"{this.initialSlug}\" to finish the test.");
+            return SelfTestStepResult.Waiting;
+        }
+
+        return SelfTestStepResult.Pass;
+    }
+
+    public void CleanUp()
+    {
+        this.initialSlug = null;
+        this.switchedAway = false;
+    }
+
+    // Any non-zero slot proves the cache was actually recomputed for the
+    // current theme. We don't compare against a reference, because custom
+    // themes can legitimately share slot values with a built-in.
+    private static bool HasPopulatedCache(Theme theme)
+    {
+        var cache = theme.AbgrCache;
+        return (cache.Primary
+              | cache.WindowBg
+              | cache.TextPrimary
+              | cache.Border) != 0u;
+    }
+}