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.

tests/test_sim2real.py

@@ -0,0 +1,39 @@
+"""Unit tests for MuJoCoRunner domain randomization."""
+
+import dataclasses
+
+import numpy as np
+import pytest
+
+from src.runners.mujoco import DomainRandConfig, MuJoCoRunnerConfig
+
+
+class TestDomainRandConfig:
+    def test_default_all_zero(self) -> None:
+        cfg = DomainRandConfig()
+        assert cfg.mass_frac == 0.0
+        assert cfg.friction_frac == 0.0
+        assert cfg.gear_frac == 0.0
+
+    def test_from_dict(self) -> None:
+        d = {"mass_frac": 0.15, "gear_frac": 0.1}
+        cfg = DomainRandConfig(**d)
+        assert cfg.mass_frac == 0.15
+        assert cfg.gear_frac == 0.1
+        assert cfg.damping_frac == 0.0  # not set
+
+
+class TestMuJoCoRunnerConfig:
+    def test_default_dr_disabled(self) -> None:
+        cfg = MuJoCoRunnerConfig()
+        assert isinstance(cfg.domain_rand, DomainRandConfig)
+        assert cfg.domain_rand.mass_frac == 0.0
+
+    def test_domain_rand_from_dict(self) -> None:
+        """Hydra passes nested configs as dicts — test __post_init__ converts."""
+        cfg = MuJoCoRunnerConfig(
+            domain_rand={"mass_frac": 0.2, "friction_frac": 0.3},  # type: ignore[arg-type]
+        )
+        assert isinstance(cfg.domain_rand, DomainRandConfig)
+        assert cfg.domain_rand.mass_frac == 0.2
+        assert cfg.domain_rand.friction_frac == 0.3