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/__init__.py

@@ -67,6 +67,7 @@ from bbctrl.AVREmu import AVREmu
 from bbctrl.IOLoop import IOLoop
 from bbctrl.MonitorTemp import MonitorTemp
 from bbctrl.Hooks import Hooks
+from bbctrl.AuxAxis import AuxAxis
 import bbctrl.Cmd as Cmd
 import bbctrl.v4l2 as v4l2
 import bbctrl.Log as log