Override USD-baked J0 stiffness; bump MAPPO entropy

The shadow_hand_instanceable_newton.usd bakes stiffness=286 / damping=57
on FFJ0/MFJ0/RFJ0/LFJ0 (MJCF→USD translation artifact). Even with the
MjcTendon constraint parsed from the MJCF source, the over-stiff joint
drive fights the tendon coupling and bounces the ball on contact.

- Add a separate ImplicitActuatorCfg ("distal_passive") on the J0 joints
  with stiffness=10.0 (1/3 of PhysX limit_stiffness=30), damping=0.1.
- Bump MAPPO entropy_loss_scale 0.0 → 0.01 to avoid premature collapse
  to the "hold ball" local optimum observed in early runs.

Author: Horde

source/isaaclab_tasks/isaaclab_tasks/direct/shadow_hand_over/agents/skrl_mappo_cfg.yaml

@@ -66,7 +66,7 @@ agent:
   ratio_clip: 0.2
   value_clip: 0.2
   clip_predicted_values: True
-  entropy_loss_scale: 0.0
+  entropy_loss_scale: 0.01
   value_loss_scale: 2.0
   kl_threshold: 0.0
   rewards_shaper_scale: 1.0

source/isaaclab_tasks/isaaclab_tasks/direct/shadow_hand_over/shadow_hand_over_env_cfg.py

@@ -195,6 +195,18 @@ def _newton_shadow_hand_cfg(
                 friction=1e-2,
                 armature=2e-3,
             ),
+            # J0 distal joints are passive in PhysX (fixed tendon, limit_stiffness=30,
+            # damping=0.1). The Newton USD bakes stiffness=286/damping=57 from the
+            # MJCF translation, which fights the MjcTendon coupling and bounces the
+            # ball. Override with stiffness=10 (1/3 of PhysX limit_stiffness=30) so
+            # the tendon constraint dominates without going fully limp.
+            "distal_passive": ImplicitActuatorCfg(
+                joint_names_expr=["robot0_(FF|MF|RF|LF)J0"],
+                stiffness=10.0,
+                damping=0.1,
+                friction=1e-2,
+                armature=2e-3,
+            ),
         },
         soft_joint_pos_limit_factor=1.0,
     )