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.

configs/runner/mujoco.yaml

@@ -2,3 +2,12 @@ num_envs: 64
 device: auto  # auto = cuda if available, else cpu
 dt: 0.002
 substeps: 10
+
+# ── Sim2real: domain randomization ───────────────────────────────
+domain_rand:
+  mass_frac: 0.15          # ±15% body mass randomization
+  friction_frac: 0.3       # ±30% joint friction
+  damping_frac: 0.3        # ±30% joint damping
+  armature_frac: 0.2       # ±20% reflected rotor inertia
+  gear_frac: 0.15          # ±15% actuator gear ratio
+  com_offset: 0.005        # ±5mm center-of-mass shift