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.

.gitignore

@@ -39,3 +39,5 @@ src/avr/emu/build/
 
 .pi/pi-python35.tar.gz
 src/py/camotics/gplan.so.built
+tmp/
+backup/