A2: name Sheen tuning knobs + SmoothStep crossfade easing

HellionChat/Ui/StyleEngine/DrawListExtensions.cs

@@ -22,6 +22,9 @@ internal static class DrawListExtensions
     // highlight, not a saturated accent flash (effect level "subtle").
     private const float SheenTintStrength = 0.35f;
 
+    // A2 tuning knob: peak alpha at t=0, fading linearly to 0 over the sweep.
+    private const byte SheenPeakAlpha = 0x40;
+
     private static readonly Dictionary<string, DateTime> SheenStarts = new();
 
     // Test-observability hook for HoverSheenAllocStep: lets the self-test
@@ -57,7 +60,7 @@ internal static class DrawListExtensions
             return;
 
         var t = (float)(elapsed / SheenDurationSeconds);
-        var alpha = (byte)Math.Round(0x40 * (1f - t));
+        var alpha = (byte)Math.Round(SheenPeakAlpha * (1f - t));
         // Tint the sweep toward the accent hue, then stamp the falloff alpha.
         // accentRgba is RGBA; convert to ABGR FIRST or the draw-list swaps R/B.
         var accentAbgr = ColourUtil.RgbaToAbgr(accentRgba);