Feat: support general single-frame cameras and add a near-horizon example

src/frame.c

@@ -413,7 +413,8 @@ int frame_lens_mesh_prepare_generation(FrameLensMesh *mesh,
         return -1;
   }
   if (config->max_level == 0)
-    return 0;
+    /* Coarse vertices still need tracing when refinement is disabled. */
+    return (int)mesh->sample_count;
   /* Every generation may batch newly inserted vertices with probes for its
    * new leaves: probe positions depend only on image-plane geometry.  Their
    * endpoints are considered only after this complete generation finishes. */