Add W axis integration via auxcnc ESP32 over /dev/ttyUSB0

Rather than rebuild gplan + the AVR firmware to add a true 7th axis,
we treat W as a synchronous out-of-band axis that moves between G-code
blocks. The pipeline:

  upload -> AuxPreprocessor rewrites W tokens into (MSG,HOOK:aux:N)
  comments -> planner sees only XYZ + messages -> Hooks fires the
  registered internal handler -> AuxAxis sends STEPS/HOME over serial
  to the ESP and blocks the planner until done.

New files:
  src/py/bbctrl/AuxAxis.py       serial worker + RPC layer
  src/py/bbctrl/AuxPreprocessor.py  G-code rewriter
  docs/AUX_W_AXIS.md             design + ops notes

Changed:
  Hooks.py        register_internal(); fix the (MSG,HOOK:...) listener
                  to read the 'messages' state list (was broken before)
  Ctrl.py         instantiate AuxAxis, register aux/aux_rel/aux_home/
                  aux_setzero hooks
  FileHandler.py  rewrite uploads in place when they use W
  Mach.py         rewrite W tokens in MDI input the same way
  Web.py          REST endpoints under /api/aux/*

The ESP firmware in ../auxcnc was extended in lockstep: HOME, HOMECFG
(NVS-persisted), WPOS, HOMED?, LIMIT?, abortable STEPS with
limit-aware abort, trapezoidal ramps, deterministic [topic] reply
tokens, [boot] banner.

Real-time decisions (limit switch, step pulses) live on the ESP. The
host owns mm units, soft limits, and aux_homed bookkeeping. ESP
reboot mid-job clears aux_homed and surfaces a message; per design
manual jogs are still allowed without homing.

src/py/bbctrl/Web.py

@@ -787,6 +787,72 @@ 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):
+        # Run synchronously via the AuxAxis' own RPC; this blocks the
+        # request. Fine because the UI shows a spinner.
+        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):
@@ -966,6 +1032,14 @@ 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'}),