AuxAxis: push home_preclear_mm via HOMECFG

Tells the auxcnc ESP how far (in steps) to back off if HOME is
invoked while the limit switch is already tripped. The ESP now
hard-fails instead of zeroing blindly when the switch stays active
after the preclear move. Default 10 mm; set home_preclear_mm=0 to
disable the preclear and revert to immediate failure.

src/py/bbctrl/AuxAxis.py

@@ -66,6 +66,14 @@ DEFAULTS = {
     'home_slow_sps':     250,     # ≈  10 mm/s
     'home_backoff_steps': 400,    # ≈  16 mm
     'home_maxtravel_steps': 200000,
+    # If HOME starts with the limit switch already tripped the ESP
+    # first moves this many steps away from the limit and then
+    # rechecks. If the switch is still active afterward, HOME hard-
+    # fails (refuses to set zero blindly when we may already be past
+    # the home position). Default ≈ 10 mm @ 25 steps/mm. Set to 0 to
+    # disable the preclear move (HOME then fails immediately if the
+    # switch reads active at start, matching the original behaviour).
+    'home_preclear_mm':  10.0,
     'step_max_sps':      4000,    # ≈ 160 mm/s normal-move cap
     'step_accel_sps2':   12000,
     'step_start_sps':    200,
@@ -500,8 +508,14 @@ class AuxAxis(object):
     def _push_homecfg(self):
         c = self._cfg
         zero_steps = self._mm_to_steps(c['home_position_mm'])
+        # preclear: how far (in steps) the ESP backs off if HOME is
+        # invoked while the limit switch is already tripped. Computed
+        # from home_preclear_mm so the operator configures it in mm.
+        spm = float(c.get('steps_per_mm', 1.0)) or 1.0
+        preclear_steps = int(round(abs(float(c['home_preclear_mm'])) * spm))
         cmd = ('HOMECFG dir=%s fast=%d slow=%d backoff=%d maxtravel=%d '
-               'zero=%d accel=%d step_max=%d step_start=%d limit_low=%d') % (
+               'zero=%d accel=%d step_max=%d step_start=%d limit_low=%d '
+               'preclear=%d') % (
             c['home_dir'],
             int(c['home_fast_sps']),
             int(c['home_slow_sps']),
@@ -512,6 +526,7 @@ class AuxAxis(object):
             int(c['step_max_sps']),
             int(c['step_start_sps']),
             1 if c['limit_low'] else 0,
+            preclear_steps,
         )
         self._rpc(cmd, topic='homecfg', timeout=3.0)