AuxAxis: ESP32-driven external stepper (auxcnc)

bbctrl.AuxAxis manages a stepper driven by an auxcnc-style ESP32
over /dev/ttyUSB0 (or whichever serial port). Persistent config in
aux.json; UI talks to it via /api/aux/* endpoints.

- AuxAxis: serial framing, position tracking, soft-limit enforcement,
  homing state machine, ATC pneumatic control (M100..M103 wrappers).
- Ctrl: instantiate self.aux alongside the other subsystems and
  close it during shutdown.
- Web: handlers for /api/aux/{config,status,home,abort,jog,move,set-zero}.

src/py/bbctrl/Ctrl.py

@@ -73,6 +73,8 @@ class Ctrl(object):
                 self.pwr = bbctrl.Pwr(self)
             with Trace.span('ctrl.hooks'):
                 self.hooks = bbctrl.Hooks(self)
+            with Trace.span('ctrl.aux'):
+                self.aux = bbctrl.AuxAxis(self)
 
             with Trace.span('ctrl.mach.connect'):
                 self.mach.connect()
@@ -130,6 +132,8 @@ class Ctrl(object):
 
 
     def close(self):
+        try: self.aux.close()
+        except Exception: pass
         self.log.get('Ctrl').info('Closing %s' % self.id)
         self.ioloop.close()
         self.avr.close()