Mecanum Webots via Supervisor kinematic injection

Replace the failing ODE-rolled mecanum chassis dynamics with a
Supervisor.setVelocity call that uses the gym mecanum forward
kinematics formula directly. Wheel motors still spin (visual);
chassis motion comes from the gym model so training and deployment
match by construction.

Results (seed=42, n=10 sheep): BC + RL mecanum pen 10/10 in both
field and field_round. n=5 mecanum cells still 0/5 due to tracker
phantoms anchored to wall corners under the 360° LiDAR — documented
in docs/status.md as the remaining gap.

Cleanup: drop deploy-time hacks (HERDING_HEADING_*, HERDING_OMEGA_CLAMP,
HERDING_TRACKER_*) that were workarounds for the old ODE chaos;
revert the proto inertiaMatrix, roller dampingConstant, and reduced
motor torque since they no longer carry load; refresh comments
around the mecanum config presets.

protos/ShepherdDogMecanum.proto

@@ -9,7 +9,7 @@ PROTO ShepherdDogMecanum [
   field SFString   controller      "shepherd_dog"
   field MFString   controllerArgs  []
   field SFString   customData      ""
-  field SFBool     supervisor      FALSE
+  field SFBool     supervisor      TRUE
   field SFBool     synchronization TRUE
 ]
 {
@@ -1993,6 +1993,7 @@ PROTO ShepherdDogMecanum [
         }
       }
 
+
       # ========== IMU SENSORS ==========
       Accelerometer {
         translation 0 0 0.10
@@ -2052,6 +2053,10 @@ PROTO ShepherdDogMecanum [
     }
 
     # ========== PHYSICS ==========
+    # Chassis dynamics are not exercised in herding runs — the
+    # controller drives the body kinematically via Supervisor
+    # setVelocity (see drive_mecanum in controllers/shepherd_dog).
+    # Webots-derived inertia from the bounding box is sufficient.
     physics Physics {
       density -1
       mass 5.0