AuxAxis: force unhome on every connect to keep host/ESP state consistent

The ESP's homed flag survives bbctrl restarts (since the ESP itself
stays powered). Host state, on the other hand, gets reset to zero on
boot - State.reset zeros ap and offset_a. Trusting the ESP's homed
flag in that situation made gplan think A was homed at machine-coord 0
while physically the axis was at 134, which then rejected any move
to the bottom (G1 A-134) as 'less than minimum soft limit 0'.

Send UNHOME (new auxcnc verb that just clears g_homed without
moving) on every host connect. The user has to re-home explicitly,
which goes through the proper Mach.home path that sets up the
offset and gplan position consistently.

Falls back to HOMED? if the firmware doesn't know UNHOME, so older
auxcnc builds keep their previous behaviour.

State.reset extended to also clear motor 4's homed flags
(<motor>homed and <motor>h) so the synthetic external-axis motor
gets reset alongside the real AVR motors.

src/py/bbctrl/AuxAxis.py

@@ -409,11 +409,28 @@ class AuxAxis(object):
             self._pos_steps = int(r.strip())
         except Exception:
             pass
+        # Force the host to start unhomed regardless of what the ESP
+        # remembers from a prior session. The ESP's homed flag survives
+        # bbctrl restarts (since the ESP itself wasn't power-cycled),
+        # but the host's planner offsets and DRO position get reset to
+        # zero on bbctrl boot. Trusting the ESP's homed flag would mean
+        # the user thinks A is homed at the wrong work-coord origin
+        # (offset_a=0 but ESP physically at home_position_mm). Sending
+        # UNHOME forces the user to re-home explicitly, which sets up
+        # the offset and gplan state correctly via the homing path in
+        # Mach.home.
         try:
-            r = self._rpc('HOMED?', topic='homed', timeout=2.0)
-            self._homed = (r.strip() == '1')
+            self._rpc('UNHOME', topic='ok', timeout=2.0)
+            self._homed = False
         except Exception:
-            pass
+            # Fall back to whatever HOMED? says - but treat any
+            # missing UNHOME support as "trust ESP's flag" so we
+            # don't break older firmware.
+            try:
+                r = self._rpc('HOMED?', topic='homed', timeout=2.0)
+                self._homed = (r.strip() == '1')
+            except Exception:
+                pass
         self._publish_state()
 
     def _reader_loop(self):

src/py/bbctrl/State.py

@@ -107,8 +107,14 @@ class State(object):
 
 
     def reset(self):
-        # Unhome all motors
-        for i in range(4): self.set('%dhomed' % i, False)
+        # Unhome all motors (real AVR motors 0..3 and the synthetic
+        # external-axis motor at index 4 used by ExternalAxis).
+        # Both <motor>homed and <motor>h are cleared - they're set
+        # by different code paths (gplan emits homed via _<axis>_homed
+        # set blocks, AVR reports h directly).
+        for i in range(5):
+            self.set('%dhomed' % i, False)
+            self.set('%dh' % i, 0)
 
         # Zero offsets and positions
         for axis in 'xyzabc':