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/Plugin.cs

@@ -339,6 +339,12 @@ public sealed class Plugin : IAsyncDalamudPlugin
                 new SelfTests.FontPushSmokeStep(this),
                 new SelfTests.WizardStateSmokeStep(this),
                 new SelfTests.FoxBannerTextureSmokeStep(this),
+                new SelfTests.SidebarModeAutoSwitchStep(this),
+                new SelfTests.ColorEditorBufferStep(this),
+                new SelfTests.ConfigMigrationV20Step(this),
+                new SelfTests.HoverSheenAllocStep(this),
+                new SelfTests.HonorificHeaderRenderStep(this),
+                new SelfTests.PerformanceBaselineStep(this),
             ]);
 
             // Re-surface the wizard for existing users when a major UX