feat(util): add ColourUtil.ApplyAlpha for hover-lerp modulation

Alpha-only modulator for ABGR colors -- RGB stays intact, factor
clamped to [0, 1]. Used by the v1.5.4 PM-3 hover-lerp path.

HellionChat/Util/ColourUtil.cs

@@ -66,6 +66,18 @@ internal static class ColourUtil
         return ((uint)a << 24) | ((uint)nb << 16) | ((uint)ng << 8) | nr;
     }
 
+    // Modulates the alpha byte of an ABGR color by a factor in [0, 1].
+    // RGB stays intact. Used by the PM-3 hover-lerp path where each
+    // frame produces a fractional alpha value but the colour itself
+    // must not shift.
+    internal static uint ApplyAlpha(uint abgr, float alphaFactor)
+    {
+        alphaFactor = Math.Clamp(alphaFactor, 0f, 1f);
+        var origAlpha = (byte)((abgr >> 24) & 0xFFu);
+        var newAlpha = (byte)Math.Round(origAlpha * alphaFactor);
+        return (abgr & 0x00FFFFFFu) | ((uint)newAlpha << 24);
+    }
+
     public static uint HexToRgba(string hex)
     {
         ArgumentNullException.ThrowIfNull(hex);