superwow: pin the bubble's seat, the one placement that could be silently wrong

The widget tests proved the frame's size, its alpha and its text; none of
them would have noticed an anchor on the wrong point, which would still
size and draw — just in the wrong place. The bubble hangs from its bottom
centre on the projected point and grows UPWARD (`0x4b0c30`), so the test
now reads `GetCenter`, `GetBottom` and `GetTop` back and pins all three.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SgstFWtn1RxEJ5kVrpgYwn

crates/benilla-ui/src/script/chat_bubble.rs

@@ -696,6 +696,26 @@ mod tests {
                  return f:GetWidth() == 120 and f:GetHeight() == 40 and f:GetAlpha() == 0.5"
             )
             .unwrap());
+        // **The seat, which is the one thing most easily wrong**: the bubble hangs from its bottom
+        // centre on the projected point and grows UPWARD (`0x4b0c30`). An anchor on the wrong
+        // point would still size and draw, just in the wrong place — and nothing else here would
+        // notice.
+        assert_eq!(
+            s.eval::<(f64, f64)>(
+                "local f = C_ChatBubbles.GetAllChatBubbles()[1]
+                 local cx = f:GetCenter()
+                 return cx, f:GetBottom()"
+            )
+            .unwrap(),
+            (400.0, 300.0),
+            "bottom centre on the seat the app handed over"
+        );
+        assert_eq!(
+            s.eval::<f64>("return C_ChatBubbles.GetAllChatBubbles()[1]:GetTop()")
+                .unwrap(),
+            340.0,
+            "and the height is added upward, not downward"
+        );
         assert!(
             s.eval::<bool>(
                 "local f = C_ChatBubbles.GetAllChatBubbles()[1]