ExternalAxis: virtual A axis through gplan, mirrored on the ESP

ExternalAxis exposes the auxcnc-driven ESP stepper as motor 4 (a
synthetic, host-only motor that gplan sees but the AVR doesn't). The
result is a virtual A axis that is fully integrated with the planner:
G1 A25 F1500 schedules a coordinated S-curve and the ESP runs the
exact same 7-segment trajectory the AVR would have run if A were a
real motor.

- ExternalAxis.py: synthetic-motor state, S-curve LINE block forward
  to the ESP, soft-limit enforcement, option-(b) homing (user A=0
  at the home limit).
- State: walk motors 0..4 in find_motor; clear both homed and h on
  reset; expose synthetic motor vars.
- axis-vars.js: motor-4 guard so the JS computed axis bindings don't
  throw when motor 4 has no entry in config.motors; resolve motor_id
  for the synthetic axis by scanning state['4an'].
- Ctrl: instantiate ExternalAxis after AuxAxis, share the axis_letter
  setting, wire AuxAxis state observer.
- Web: route /api/aux/{home,jog,move} through ExternalAxis when it
  is enabled so the DRO and synthetic-motor flags stay in sync.

src/py/bbctrl/__init__.py

@@ -68,6 +68,7 @@ from bbctrl.IOLoop import IOLoop
 from bbctrl.MonitorTemp import MonitorTemp
 from bbctrl.Hooks import Hooks
 from bbctrl.AuxAxis import AuxAxis
+from bbctrl.ExternalAxis import ExternalAxis
 import bbctrl.Cmd as Cmd
 import bbctrl.v4l2 as v4l2
 import bbctrl.Log as log