fix(contact): align Mechanism panel with Propagation Analysis ducts

The radar-only RainScatterClassifier sees ducting only when HRRR's
pressure-level M-profile flagged it. Sounding-detected and gradient-
inferred ducts surface in elevation_profile.ducts via the analysis
pipeline — promote those into :tropo_duct so the Mechanism badge
agrees with the narrative.

lib/microwaveprop_web/live/contact_live/show.ex

@@ -624,13 +624,31 @@ defmodule MicrowavepropWeb.ContactLive.Show do
         elevation_profile
       )
 
+    mechanism = upgrade_mechanism(socket.assigns[:mechanism], elevation_profile)
+
     assign(socket,
       elevation_profile: elevation_profile,
       propagation_analysis: propagation_analysis,
-      data_sources: data_sources
+      data_sources: data_sources,
+      mechanism: mechanism
     )
   end
 
+  # The radar-only RainScatterClassifier sees ducting only when HRRR's
+  # pressure-level M-profile already flagged it. Sounding and gradient-
+  # inferred ducts surface in elevation_profile.ducts via the analysis
+  # pipeline — promote those into :tropo_duct so the Mechanism panel
+  # agrees with the Propagation Analysis narrative.
+  defp upgrade_mechanism(nil, _profile), do: nil
+  defp upgrade_mechanism(:tropo_duct, _profile), do: :tropo_duct
+  defp upgrade_mechanism(:likely_rainscatter, _profile), do: :likely_rainscatter
+
+  defp upgrade_mechanism(mechanism, %{ducts: [_ | _] = ducts}) do
+    if Enum.any?(ducts, &Map.get(&1, :likely)), do: :tropo_duct, else: mechanism
+  end
+
+  defp upgrade_mechanism(mechanism, _profile), do: mechanism
+
   defp load_iemre(contact) do
     case Weather.iemre_for_contact(contact) do
       %{hourly: hourly} = obs when is_list(hourly) and hourly != [] ->