set a flag when Z stop is set

As I see the fluidnc code, homing goes to the limit switches, which
the maslow wants to use when setting where the bit will cut. But
since the maslow belts need to be at different lengths based on the Z
distance from the mechanical limits, knowing if that has been set
requires an additional flag.

it is set false by default but set true when you manually set the
mechanical limit or when you restore the position.

FluidNC/src/Maslow/Maslow.cpp

@@ -1405,6 +1405,12 @@ void Maslow_::runCalibration() {
     }
     stop();
 
+    //if the Z mechanical limit is not known, we can't run calibration
+    if (!zMechanicalLimitSet) {
+        log_error("Cannot run calibration until the Z position is known. Run it down to the stops and set Z stop");
+        sys.set_state(State::Idle);
+        return;
+    }
     //if not all axis are homed, we can't run calibration, OR if the user hasnt entered width and height?
     if (!allAxisExtended()) {
         log_error("Cannot run calibration until all belts are extended fully");
@@ -1627,6 +1633,7 @@ void Maslow_::loadZPos() {
         int zAxis = 2;
         float* mpos = get_mpos();
         mpos[zAxis] = targetZ;
+        zMechanicalLimitSet = true;
         set_motor_steps_from_mpos(mpos);
 
         log_info("Current z-axis position loaded as: " << targetZ);
@@ -1645,6 +1652,7 @@ void Maslow_::setZStop() {
     int zAxis = 2;
     float* mpos = get_mpos();
     mpos[zAxis] = targetZ;
+    zMechanicalLimitSet = true;
     set_motor_steps_from_mpos(mpos);
 
     gc_sync_position();//This updates the Gcode engine with the new position from the stepping engine that we set with set_motor_steps

FluidNC/src/Maslow/Maslow.h

@@ -64,6 +64,7 @@ struct TelemetryData {
     bool extendedTR;
     bool extendedBL;
     bool extendedBR;
+    bool zMechanicalLimitSet = false; //This is separate from homing because this is the mechanical stop of the machine while Z home is where the bit is at Z=0
 
     bool extendingALL;
     bool complyALL;