Radar: nearby_objects every second and dungeon_map per dungeon landblock

start_radar/stop_radar from the backend drive it, as before. nearby_objects
keeps the original's shape: map coordinates on the surface, landblock-local
raw positions with zero map coordinates in a dungeon, ids as signed ints, the
same eight object classes, no wielded items or self. dungeon_map is built the
original's way from the landblock's cells (x a multiple of 10, whole metres,
6-metre floors, rotation = the orientation's stored W) and sent once per
landblock per session; the cells come through a small reader that the
plugin API's dungeon-cell data will fill (until then the map is not sent).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/OpenAC.MosswartMassacre/FeatureCatalog.cs

@@ -36,5 +36,16 @@ internal static class FeatureCatalog
         session.AddConnectionListener(sharing);
         session.AddShareHandler(sharing);
         features.Add(new Sharing.VitalSharingOverlay(context, sharing));
+
+        // The radar the backend turns on and off.
+        var radar = new Radar.RadarStream(context, cells: null);
+        features.Add(radar);
+        session.RadarToggle = on =>
+        {
+            if (on)
+                radar.Start();
+            else
+                radar.Stop();
+        };
     }
 }