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/Web.py

@@ -787,6 +787,70 @@ class HooksFireHandler(bbctrl.APIHandler):
         self.get_ctrl().hooks._fire(event, data)
 
 
+# ----- W axis (auxcnc) endpoints --------------------------------------------
+
+class AuxConfigGetHandler(bbctrl.APIHandler):
+    def get(self):
+        self.write_json(self.get_ctrl().aux.get_config())
+
+
+class AuxConfigSaveHandler(bbctrl.APIHandler):
+    def put_ok(self):
+        self.get_ctrl().aux.save_config(self.json or {})
+
+
+class AuxStatusHandler(bbctrl.APIHandler):
+    def get(self):
+        aux = self.get_ctrl().aux
+        self.write_json({
+            'enabled':  aux.enabled,
+            'present':  aux.present,
+            'homed':    aux.homed,
+            'pos_mm':   aux.position_mm,
+        })
+
+
+class AuxHomeHandler(bbctrl.APIHandler):
+    def put_ok(self):
+        self.get_ctrl().aux.home()
+
+
+class AuxAbortHandler(bbctrl.APIHandler):
+    def put_ok(self):
+        self.get_ctrl().aux.abort()
+
+
+class AuxJogHandler(bbctrl.APIHandler):
+    """Body: {"mm": 1.5}  for relative-mm move,
+             {"steps": 200} for raw step move (bypasses soft limits)."""
+    def put_ok(self):
+        body = self.json or {}
+        aux = self.get_ctrl().aux
+        if 'mm' in body:
+            aux.move_rel_mm(float(body['mm']))
+        elif 'steps' in body:
+            aux.jog_steps(int(body['steps']))
+        else:
+            raise HTTPError(400, 'mm or steps required')
+
+
+class AuxMoveHandler(bbctrl.APIHandler):
+    """Body: {"mm": 12.5}  absolute move in mm."""
+    def put_ok(self):
+        body = self.json or {}
+        if 'mm' not in body:
+            raise HTTPError(400, 'mm required')
+        self.get_ctrl().aux.move_abs_mm(float(body['mm']))
+
+
+class AuxSetZeroHandler(bbctrl.APIHandler):
+    """Body: {"mm": 0}  set current position to <mm>."""
+    def put_ok(self):
+        body = self.json or {}
+        mm = float(body.get('mm', 0.0))
+        self.get_ctrl().aux.set_position_mm(mm)
+
+
 class RemoteDiagnosticsHandler(bbctrl.APIHandler):
 
     def get(self):
@@ -1016,6 +1080,14 @@ class Web(tornado.web.Application):
             (r'/api/hooks/save', HooksSaveHandler),
             (r'/api/hooks/status', HooksStatusHandler),
             (r'/api/hooks/fire/([\w-]+)', HooksFireHandler),
+            (r'/api/aux/config', AuxConfigGetHandler),
+            (r'/api/aux/config/save', AuxConfigSaveHandler),
+            (r'/api/aux/status', AuxStatusHandler),
+            (r'/api/aux/home', AuxHomeHandler),
+            (r'/api/aux/abort', AuxAbortHandler),
+            (r'/api/aux/jog', AuxJogHandler),
+            (r'/api/aux/move', AuxMoveHandler),
+            (r'/api/aux/set-zero', AuxSetZeroHandler),
             (r'/(.*)', StaticFileHandler,
              {'path': bbctrl.get_resource('http/'),
               'default_filename': 'index.html'}),