motion/control: guard jerk decomposition for identity kinematics only

The S-curve jerk decomposition assumed joint[0,1,2] = X,Y,Z, which is
only true for identity kinematics. For KINEMATICS_BOTH (e.g. XXYYYZ or
rotatekins) joint indices don't map to axis letters, making the
decomposition incorrect. Guard with KINEMATICS_IDENTITY check so
non-trivial kinematics fall back to cubic interpolator jerk values.

Author: Luca Toniolo

src/emc/motion/control.c

@@ -1398,21 +1398,17 @@ static void get_pos_cmds(long period)
 		joint->pos_cmd = cubicInterpolate(&(joint->cubic), 0, &(joint->vel_cmd), &(joint->acc_cmd),  &(joint->jerk_cmd));
 	}
 
-	/* Use accurate jerk values from TP output (for Cartesian machines only)
-	 * For standard XYZ machines, joint[0-2] correspond to X, Y, Z axes
-	 * TP outputs: current_jerk (path jerk) and current_dir (direction unit vector)
-	 * Per-axis jerk = path_jerk * direction_component
+	/* Use accurate jerk values from TP output for identity kinematics only.
+	 * For KINEMATICS_BOTH (non-trivial joint mapping), joint indices don't
+	 * necessarily correspond to XYZ axes, so keep cubic interpolator values.
 	 */
-	if (emcmotStatus->planner_type == 1) {
-	    // S-curve mode: use accurate jerk values
+	if (emcmotStatus->planner_type == 1
+	    && emcmotConfig->kinType == KINEMATICS_IDENTITY) {
 	    double path_jerk = emcmotStatus->current_jerk;
 	    PmCartesian dir = emcmotStatus->current_dir;
-
-	    // For the first 3 joints (assuming X, Y, Z), use accurate jerk
 	    if (NO_OF_KINS_JOINTS >= 1) joints[0].jerk_cmd = path_jerk * dir.x;
 	    if (NO_OF_KINS_JOINTS >= 2) joints[1].jerk_cmd = path_jerk * dir.y;
 	    if (NO_OF_KINS_JOINTS >= 3) joints[2].jerk_cmd = path_jerk * dir.z;
-	    // Rotary axes (A, B, C) keep the cubic interpolator values for now
 	}
 
 	/* report motion status */