Fix: move tracking camera inside worldbody element

MuJoCo requires camera elements to be inside worldbody, not at the top level.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/drone_forest.xml

@@ -221,6 +221,10 @@
       <geom name="canopy_24" type="sphere" size="0.6" pos="0 0 3.2"
             material="canopy_mat" contype="0" conaffinity="0"/>
     </body>
+
+    <!-- Overhead tracking camera -->
+    <camera name="tracking" pos="0 0 25" xyaxes="1 0 0 0 1 0" mode="trackcom"
+            fovy="60"/>
   </worldbody>
 
   <!-- Site-based thrust actuators: force applied in site-local z (tilts with drone) -->
@@ -234,8 +238,4 @@
     <general name="thrust_bl" site="rotor_bl" gear="0 0 1 0 0 0" ctrllimited="true"
              ctrlrange="0 3" gainprm="1" biasprm="0 0 0"/>
   </actuator>
-
-  <!-- Overhead tracking camera -->
-  <camera name="tracking" pos="0 0 25" xyaxes="1 0 0 0 1 0" mode="trackcom"
-          fovy="60"/>
 </mujoco>