Fix incorrect layer number in G-code conflict warning (#16588)

Fix layer number in G-code conflict warning

Correct the comparison used to look up a layer by Z height.

The reversed comparison could return the total layer count instead of
the layer corresponding to the reported conflict height.

src/libvgcode/src/Layers.cpp

@@ -75,7 +75,7 @@ std::vector<float> Layers::get_zs() const
 
 size_t Layers::get_layer_id_at(float z) const
 {
-    auto iter = std::upper_bound(m_items.begin(), m_items.end(), z, [](float z, const Item& item) { return item.z < z; });
+    auto iter = std::upper_bound(m_items.begin(), m_items.end(), z, [](float z, const Item& item) { return item.z > z; });
     return std::distance(m_items.begin(), iter);
 }