Apply 6-degree IMU pitch offset correction for G1 hardware

G1's physical IMU is mounted with ~6° forward pitch bias. SONIC's NN
was trained with a perfect virtual IMU, causing backward lean compensation.
Pre-rotate base_quat by -6° about Y axis before it enters observations.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

gear_sonic_deploy/src/g1/g1_deploy_onnx_ref/src/g1_deploy_onnx_ref.cpp

@@ -2653,7 +2653,13 @@ class G1Deploy {
         }
       }
 
-      std::array<double, 4> base_quat = float_to_double<4>(ls->imu_state().quaternion()); // qw, qx, qy, qz
+      std::array<double, 4> base_quat_raw = float_to_double<4>(ls->imu_state().quaternion()); // qw, qx, qy, qz
+      // IMU pitch correction: G1's IMU is mounted with ~6° forward pitch offset.
+      // The NN was trained with a perfect IMU, so we pre-rotate by -6° about Y axis
+      // to correct the bias before it enters observations (gravity dir, heading, etc.).
+      constexpr double IMU_PITCH_OFFSET_RAD = -6.0 * M_PI / 180.0;  // negative = correct forward lean
+      static const auto imu_pitch_correction = quat_from_angle_axis_d(IMU_PITCH_OFFSET_RAD, {0.0, 1.0, 0.0});
+      std::array<double, 4> base_quat = quat_mul_d(imu_pitch_correction, base_quat_raw);
       std::array<double, 3> base_ang_vel = float_to_double<3>(ls->imu_state().gyroscope());
       std::array<double, 3> base_accel = float_to_double<3>(ls->imu_state().accelerometer());