feat(ui): wire ThemeRegistry crossfade into PushGlobal

Switch picks a lerped AbgrCache during the 300ms crossfade window
(ReduceMotion bypass keeps the snap path). Plugin-load init path
switches to SwitchSilent so opening the plugin no longer fades from
the default theme. WindowBg/ChildBg RGBA path stays bound to the
user's per-window opacity override and never fades.

PushGlobal takes the ThemeRegistry as a parameter -- it is an instance
member on Plugin, not static, so the single Plugin.Draw call-site
threads it through alongside the active theme.

HellionChat/Plugin.cs

@@ -912,6 +912,7 @@ public sealed class Plugin : IAsyncDalamudPlugin
         // Theme engine is always active; Classic is a theme, not a disabled state.
         using IDisposable _style = HellionStyle.PushGlobal(
             ThemeRegistry.Active,
+            ThemeRegistry,
             Config.WindowOpacity
         );