refactor: merge motor sysid into unified sysid module

Unified the two separate sysid codepaths (motor-only and full-system)
into a single module that optimizes all 28 parameters jointly:

- 13 motor params (asymmetric gear, damping, friction, deadzone,
  Stribeck boost, action bias, filter tau, armature, ctrl_limit)
- 15 pendulum/arm params (mass, CoM, inertia, joint dynamics)

Key changes:
- Added stribeck_friction_boost, stribeck_vel, action_bias to
  ActuatorConfig (robot.py) and MJX runner
- Created shared src/sysid/preprocess.py (SG velocity recomputation)
- Rewrote src/sysid/rollout.py with unified MOTOR_PARAMS + PENDULUM_PARAMS
  spec and PARAM_SETS dict for flexible subset optimization
- Updated optimize.py, export.py, visualize.py to use unified params
  (removed all LOCKED_MOTOR_PARAMS references)
- Removed src/sysid/motor/ module and scripts/motor_sysid.py

Net: -1383 lines, zero code duplication between motor and full-system sysid.

assets/rotary_cartpole/robot.yaml

@@ -1,21 +1,21 @@
-# Tuned robot config — generated by src.sysid.optimize
+# Tuned robot config — generated by src.sysid (2026-03-28)
+# Motor params: motor-only sysid cost 0.2117
+# Full-system params: sysid cost 1.216 (0.2s windows, amplitude 200)
 
 urdf: rotary_cartpole.urdf
 actuators:
 - joint: motor_joint
   type: motor
-  gear: [0.424182, 0.425031]          # torque constant [pos, neg] (motor sysid)
-  ctrl_range: [-0.592, 0.592]         # effective control bound (sysid-tuned)
-  deadzone: [0.141291, 0.078015]      # L298N min |ctrl| for torque [pos, neg]
-  damping: [0.002027, 0.014665]       # viscous damping [pos, neg]
-  frictionloss: [0.057328, 0.053355]  # Coulomb friction [pos, neg]
-  filter_tau: 0.005035                # 1st-order actuator filter (motor sysid)
-  viscous_quadratic: 0.000285         # velocity² drag
-  back_emf_gain: 0.006758             # back-EMF torque reduction
+  gear: [0.371194, 0.428143]          # torque constant [pos, neg] (motor sysid)
+  ctrl_range: [-0.611479, 0.611479]   # effective control bound (full sysid)
+  deadzone: [0.141820, 0.031454]      # L298N min |ctrl| for torque [pos, neg]
+  damping: [0.001384, 0.005196]       # viscous damping [pos, neg]
+  frictionloss: [0.036744, 0.069082]  # Coulomb friction [pos, neg]
+  filter_tau: 0.022301                # 1st-order actuator filter (motor sysid)
 joints:
   motor_joint:
-    armature: 0.002773                # reflected rotor inertia (motor sysid)
+    armature: 0.002753                # reflected rotor inertia (motor sysid)
     frictionloss: 0.0                 # handled by motor model via qfrc_applied
   pendulum_joint:
-    damping: 0.000119
-    frictionloss: 1.0e-05
+    damping: 1.3e-06                  # full sysid (was 0.000119)
+    frictionloss: 3.7e-06             # full sysid (was 1.0e-05)