test(selftests): add v1.6.0 SelfTest steps from sub-spec

Six new ISelfTestStep entries register with the Dalamud SelfTestRegistry:
SidebarModeAutoSwitch probes the width threshold both above and below the
exact boundary so the >= contract stays pinned; ColorEditorBuffer is a
placeholder until the picker arrives; ConfigMigrationV20 asserts the
schema stamp and the five v20 field defaults; HoverSheenAlloc drives 100
hovered frames against three constant keys and a final un-hover sweep to
exercise the cleanup branch; HonorificHeaderRender runs one Draw call on
the live component to catch IPC fallback crashes; PerformanceBaseline
prints a JSON line of the IO counters so the cycle notes can pick up a
snapshot. MainWindow gets internal accessors so the probes can reach the
sidebar and honorific header without widening the public surface.

HellionChat/Ui/Windows/MainWindow.cs

@@ -57,6 +57,12 @@ internal sealed class MainWindow : Window
 
     public Tab? ActiveTab => _activeTab;
 
+    // Internal accessors for self-tests so the probes can reach the live
+    // component without exposing them as public surface.
+    internal Components.Sidebar GetSidebarForSelfTest() => _sidebar;
+
+    internal Components.HonorificHeader GetHonorificHeaderForSelfTest() => _honorific;
+
     // new-shadow on Window.Toggle so the open path also writes Config —
     // OnClose already covers the close path through the base behaviour.
     public new void Toggle()