Pointer: don't put a vanished panel back in the visibility map

The panel-edge test read visible[edgeKey], and when the last panel the cursor touched was
gone from the overlay list, that added it back as hidden. The map then had more entries than
there are handles, which made the 50 ms visibility poll run every frame, and since the last
commit it also counted as a visibility change each time, so unchanged frames were never
reused. The edge test now looks the key up without adding it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pointer/helper/ft-pointer.cpp

@@ -2183,7 +2183,8 @@ int main() {
                 onEdge = false;
                 if (!dragging && best < 1e8 && !bestScene) {
                     edgeKey = bestKey, edgePoint = bestPoint, edgeNormal = Normalize(bestNormal), edgeLast = bestPoint;
-                } else if (!dragging && best >= 1e8 && !edgeKey.empty() && visible[edgeKey]) {
+                } else if (!dragging && best >= 1e8 && !edgeKey.empty() && visible.count(edgeKey) &&
+                           visible.at(edgeKey)) {
                     // Just off a panel: stay on its plane (see "Panel edges" at the top).
                     const double denom = Dot(dir, edgeNormal);
                     if (std::fabs(denom) > 1e-4) {