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.

scripts/motor_sysid.py

@@ -1,64 +0,0 @@
-"""Unified CLI for motor-only system identification.
-
-Usage:
-    python scripts/motor_sysid.py capture   --duration 20
-    python scripts/motor_sysid.py optimize  --recording assets/motor/recordings/<file>.npz
-    python scripts/motor_sysid.py visualize --recording assets/motor/recordings/<file>.npz
-    python scripts/motor_sysid.py export    --result assets/motor/motor_sysid_result.json
-"""
-
-from __future__ import annotations
-
-import sys
-from pathlib import Path
-
-# Ensure project root is on sys.path
-_PROJECT_ROOT = str(Path(__file__).resolve().parent.parent)
-if _PROJECT_ROOT not in sys.path:
-    sys.path.insert(0, _PROJECT_ROOT)
-
-
-def main() -> None:
-    if len(sys.argv) < 2 or sys.argv[1] in ("-h", "--help"):
-        print(
-            "Motor System Identification\n"
-            "===========================\n"
-            "Usage: python scripts/motor_sysid.py <command> [options]\n"
-            "\n"
-            "Commands:\n"
-            "  capture    Record motor trajectory under PRBS excitation\n"
-            "  optimize   Run CMA-ES to fit motor parameters\n"
-            "  visualize  Plot real vs simulated motor response\n"
-            "  export     Write tuned MJCF + robot.yaml files\n"
-            "\n"
-            "Workflow:\n"
-            "  1. Flash sysid firmware to ESP32 (motor-only, no limits)\n"
-            "  2. python scripts/motor_sysid.py capture --duration 20\n"
-            "  3. python scripts/motor_sysid.py optimize --recording <file>.npz\n"
-            "  4. python scripts/motor_sysid.py visualize --recording <file>.npz\n"
-            "\n"
-            "Run '<command> --help' for command-specific options."
-        )
-        sys.exit(0)
-
-    command = sys.argv[1]
-    sys.argv = [f"motor_sysid {command}"] + sys.argv[2:]
-
-    if command == "capture":
-        from src.sysid.motor.capture import main as cmd_main
-    elif command == "optimize":
-        from src.sysid.motor.optimize import main as cmd_main
-    elif command == "visualize":
-        from src.sysid.motor.visualize import main as cmd_main
-    elif command == "export":
-        from src.sysid.motor.export import main as cmd_main
-    else:
-        print(f"Unknown command: {command}")
-        print("Available commands: capture, optimize, visualize, export")
-        sys.exit(1)
-
-    cmd_main()
-
-
-if __name__ == "__main__":
-    main()