feat: add FrameLerp helper and per-tab hover-alpha fields

FrameLerp.Smooth is the framerate-independent smoothing path -- a
Umbra-style v += (target - v) * factor with the factor clamped to 1
so a stalled frame snaps cleanly instead of overshooting. Tab gets
two NonSerialized fields (_hoverAlpha, _cardHoverAlpha) that the
v1.5.4 render loops drive.

HellionChat/Configuration.cs

@@ -485,6 +485,17 @@ public class Tab
     [NonSerialized]
     internal string? _cachedTellIcon;
 
+    // PM-3 hover-lerp state. Default 0f means "not hovered". Sidebar
+    // path animates per tab; card-mode-border path is tab-aggregate
+    // (any card-row hover ramps the alpha for all cards in this tab).
+    // Lerp speed lives in the render loop, not here, so the same field
+    // serves both sites at the same animation curve.
+    [NonSerialized]
+    internal float _hoverAlpha;
+
+    [NonSerialized]
+    internal float _cardHoverAlpha;
+
     public bool Matches(Message message)
     {
         if (!message.Matches(SelectedChannels, ExtraChatAll, ExtraChatChannels))

HellionChat/Util/FrameLerp.cs

@@ -0,0 +1,17 @@
+namespace HellionChat.Util;
+
+// Framerate-independent smoothing for per-frame hover and motion
+// values. Pattern anchor: Umbra Toolbar.Autohide.cs:55
+// (`v += (target - v) * deltaTime`). The Math.Min(1f, speed*dt)
+// clamp is a deliberate HellionChat addition -- on Wine/DXVK a
+// stalled frame can push deltaTime well over 16ms, which would
+// otherwise let the raw factor exceed 1.0 and overshoot the target.
+// Clamping makes a stalled frame land exactly on target instead.
+internal static class FrameLerp
+{
+    public static float Smooth(float current, float target, float speed, float deltaTime)
+    {
+        var factor = Math.Min(1f, speed * deltaTime);
+        return current + (target - current) * factor;
+    }
+}