tame the reply swipe so vertical scrolling stays clean

- Raise the swipe dominance threshold to 2.0 and add a 24pt
  activation distance so diagonal timeline scrolls never move bubbles
  or fire haptics, and snap the indicator back when a gesture turns
  vertical mid-drag.
- Cover the stricter thresholds in MessageReplySwipeTests.

Sources/Shared/Models/MessageReplySwipePolicy.swift

@@ -2,12 +2,19 @@ import CoreGraphics
 
 struct MessageReplySwipePolicy {
     static let triggerDistance: CGFloat = 64
+    /// Horizontal travel required before the reply indicator appears. Keeps
+    /// the per-cell state untouched during ordinary vertical scrolling.
+    static let activationDistance: CGFloat = 24
     static let maximumOffset: CGFloat = 76
-    static let horizontalDominance: CGFloat = 1.4
+    /// The drag must be clearly horizontal: a mostly vertical timeline scroll
+    /// (even a diagonal one) must never arm the reply swipe or move bubbles.
+    static let horizontalDominance: CGFloat = 2.0
 
     static func offset(for translation: CGSize) -> CGFloat {
         guard isHorizontal(translation) else { return 0 }
-        let distance = min(abs(translation.width) * 0.72, maximumOffset)
+        let width = abs(translation.width)
+        guard width >= activationDistance else { return 0 }
+        let distance = min((width - activationDistance) * 0.72 + 12, maximumOffset)
         return translation.width < 0 ? -distance : distance
     }