fix: add __changed__ to test socket assigns for LiveView assign/3 compatibility

When cancel_pending_loads now succeeds (after fixing the pending_batch_tasks
KeyError), it calls Phoenix.LiveView.assign/3 which requires __changed__ in
the socket assigns. The test socket was created as a bare %Socket{} without
this internal LiveView field.

test/aprsme_web/live/map_live/display_manager_test.exs

@@ -7,6 +7,7 @@ defmodule AprsmeWeb.MapLive.DisplayManagerTest do
   setup do
     socket = %Socket{
       assigns: %{
+        __changed__: %{},
         visible_packets: %{},
         tracked_callsign: nil,
         packet_age_threshold: nil,
@@ -43,6 +44,7 @@ defmodule AprsmeWeb.MapLive.DisplayManagerTest do
   # Builds from known base defaults so callers can pass only overrides.
   defp update_assigns(socket, overrides) do
     base = %{
+      __changed__: %{},
       visible_packets: %{},
       tracked_callsign: nil,
       packet_age_threshold: nil,