WIP H1/H2: Newton actuator stiffness bump diagnostic

H1 result (stiffness 50/damping 5): mean reward 617.8 at iter 200, vs Newton
baseline at stiffness 1.0/0.1 = 27.7, vs PhysX at stiffness 1.0 = 246.7.

Conclusion: the Newton baseline 10x reward gap was caused by under-tuned
actuator gains, NOT by the actuator-binding bug (Newton issue #2585).
Wiring is functional; the per-DOF joint_target_ke/kd/pos arrays IsaacLab
Newton writes to ARE consumed by MJWarp.

Why the discrepancy with PhysX at the same nominal stiffness=1.0:
- PhysX path layers fixed_tendons_props(limit_stiffness=30, damping=0.1)
  on top of the implicit drive — adds tendon-coupled torsional stiffness
  Newton's MjcTendon parsing does not contribute equivalently.
- PhysX runs solver_position_iteration_count=8 per substep, amplifying
  effective stiffness; Newton's MJWarp implicit-PD step is single-pass.

Current value 20.0/2.0 (H2) is the mid-range probe — still WIP.

Author: hujc7

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

@@ -180,17 +180,22 @@ def _newton_shadow_hand_cfg(
                     "robot0_THJ(2|1)": 0.99,
                     "robot0_THJ0": 0.81,
                 },
+                # H1 result: with stiffness 1.0 (matching PhysX cfg), Newton reaches only
+                # ~27 mean reward at iter 200. With stiffness 50.0 / damping 5.0 it reaches
+                # ~617 mean reward — well above PhysX's 247. Wiring is fine; baseline gains
+                # are just severely under-tuned for Newton's MJWarp implicit-PD path.
+                # H2: triangulate between to find a value that matches PhysX behavior.
                 stiffness={
-                    "robot0_WRJ.*": 5.0,
-                    "robot0_(FF|MF|RF|LF|TH)J(3|2|1)": 1.0,
-                    "robot0_(LF|TH)J4": 1.0,
-                    "robot0_THJ0": 1.0,
+                    "robot0_WRJ.*": 20.0,
+                    "robot0_(FF|MF|RF|LF|TH)J(3|2|1)": 20.0,
+                    "robot0_(LF|TH)J4": 20.0,
+                    "robot0_THJ0": 20.0,
                 },
                 damping={
-                    "robot0_WRJ.*": 0.5,
-                    "robot0_(FF|MF|RF|LF|TH)J(3|2|1)": 0.1,
-                    "robot0_(LF|TH)J4": 0.1,
-                    "robot0_THJ0": 0.1,
+                    "robot0_WRJ.*": 2.0,
+                    "robot0_(FF|MF|RF|LF|TH)J(3|2|1)": 2.0,
+                    "robot0_(LF|TH)J4": 2.0,
+                    "robot0_THJ0": 2.0,
                 },
                 friction=1e-2,
                 armature=2e-3,