A2: name Sheen tuning knobs + SmoothStep crossfade easing

HellionChat/Themes/ThemeRegistry.cs

@@ -522,7 +522,10 @@ public sealed class ThemeRegistry
         )
         {
             var t = (float)(now - _crossfadeStartTickMs) / CrossfadeDurationMs;
-            snapshot = ThemeAbgrCacheLerp.Lerp(_previousAbgrSnapshot.Value, _active.AbgrCache, t);
+            // A2: SmoothStep easing so the fade eases in/out instead of a
+            // linear ramp. MUST stay in lockstep with TryGetActiveCrossfade (K8).
+            var te = t * t * (3f - 2f * t);
+            snapshot = ThemeAbgrCacheLerp.Lerp(_previousAbgrSnapshot.Value, _active.AbgrCache, te);
         }
         else
         {
@@ -548,7 +551,9 @@ public sealed class ThemeRegistry
             return false;
 
         var t = (float)elapsed / CrossfadeDurationMs;
-        lerped = ThemeAbgrCacheLerp.Lerp(_previousAbgrSnapshot.Value, _active.AbgrCache, t);
+        // A2: SmoothStep easing -- keep identical to ArmCrossfade (K8).
+        var te = t * t * (3f - 2f * t);
+        lerped = ThemeAbgrCacheLerp.Lerp(_previousAbgrSnapshot.Value, _active.AbgrCache, te);
         return true;
     }
 

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);