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/Ctrl.py

@@ -75,6 +75,19 @@ class Ctrl(object):
                 self.hooks = bbctrl.Hooks(self)
             with Trace.span('ctrl.aux'):
                 self.aux = bbctrl.AuxAxis(self)
+            with Trace.span('ctrl.ext_axis'):
+                # ExternalAxis exposes the auxcnc ESP stepper as a
+                # virtual A axis that gplan handles natively. Created
+                # unconditionally so State sees the synthetic motor
+                # vars even when aux is disabled (kept inert in that
+                # case via ext_axis.enabled).
+                axis_letter = self.aux._cfg.get('axis_letter', 'a')
+                self.ext_axis = bbctrl.ExternalAxis(
+                    self, self.aux, axis_letter=axis_letter)
+                # Hook AuxAxis post-publish callback so homed flag
+                # mirrors into State after homing.
+                self.aux.set_state_observer(
+                    self.ext_axis.refresh_homed)
 
             with Trace.span('ctrl.mach.connect'):
                 self.mach.connect()
@@ -132,6 +145,8 @@ class Ctrl(object):
 
 
     def close(self):
+        try: self.ext_axis.close()
+        except Exception: pass
         try: self.aux.close()
         except Exception: pass
         self.log.get('Ctrl').info('Closing %s' % self.id)