refactor(theme): extract ChildBgAlpha threshold logic to testable helper (F1.2)

HellionStyle.PushGlobal had two lines that resolved the child-bg alpha
based on window opacity. Moves the 0.999f threshold and the alpha-mask
into HellionStyleHelpers.ResolveChildBgAlpha so the logic is reachable
from the build suite without touching the ImGui surface.

HellionChat/Ui/HellionStyle.cs

@@ -43,13 +43,10 @@ internal static class HellionStyle
         var alphaByte = (uint)Math.Clamp((int)(windowOpacity * 255f), 0x55, 0xFF);
         var windowBgWithAlpha = (c.WindowBg & 0xFFFFFF00u) | alphaByte;
 
-        // ChildBg alpha: child areas rendered inside ChatLogWindow would
-        // multiply their alpha with WindowBg, making 50% opacity appear
-        // ~75% solid. At full opacity the theme's alpha is preserved; below
-        // it ChildBg goes fully transparent so only WindowBg sets the final
-        // coverage.
-        var childBgAlpha = windowOpacity >= 0.999f ? (c.ChildBg & 0xFFu) : 0u;
-        var childBgWithAlpha = (c.ChildBg & 0xFFFFFF00u) | childBgAlpha;
+        // ChildBg alpha resolution lives in HellionStyleHelpers so the
+        // threshold logic can be covered by a pure-helper test in the
+        // build suite.
+        var childBgWithAlpha = HellionStyleHelpers.ResolveChildBgAlpha(c.ChildBg, windowOpacity);
 
         // Layout
         stack.PushStyleVar(ImGuiStyleVar.WindowRounding, l.WindowRounding);