AuxAxis: faster sane defaults for homing and stepping

Old defaults (4000 fast / 400 slow / 200 backoff / 16000 accel /
200 start) were never aggressive in practice because the user-
saved config drifted to even lower values (600/80/110). Re-tune
the DEFAULTS dict to values that are sensible at 25 steps/mm:

  home_fast_sps     2500   ~100 mm/s seek
  home_slow_sps      250    ~10 mm/s re-engage
  home_backoff_steps 400     ~16 mm clear hysteresis
  step_max_sps      4000   ~160 mm/s normal-move cap
  step_accel_sps2  12000

These only affect machines without an existing aux.json. The Pi
at 10.1.10.55 was patched manually.

src/py/bbctrl/AuxAxis.py

@@ -41,12 +41,17 @@ DEFAULTS = {
     'home_dir':          '-',     # which direction is "toward limit" (host's view)
     'home_position_mm':  0.0,     # mm value to assign at home
     # ESP-side homing rates (steps/sec). Pushed via HOMECFG on connect.
-    'home_fast_sps':     4000,
+    # Speeds tuned for a typical 25 steps/mm aux drive (so 1 step =
-    'home_slow_sps':     400,
+    # 0.04 mm). With the limit-aware ESP firmware these values give
-    'home_backoff_steps': 200,
+    # a brisk seek (100 mm/s), enough backoff to clear the switch
+    # hysteresis (16 mm), and a slow re-engage (10 mm/s) that's
+    # accurate without being painfully slow on a longer axis.
+    'home_fast_sps':     2500,    # ≈ 100 mm/s @ 25 steps/mm
+    'home_slow_sps':     250,     # ≈  10 mm/s
+    'home_backoff_steps': 400,    # ≈  16 mm
     'home_maxtravel_steps': 200000,
-    'step_max_sps':      4000,
+    'step_max_sps':      4000,    # ≈ 160 mm/s normal-move cap
-    'step_accel_sps2':   16000,
+    'step_accel_sps2':   12000,
     'step_start_sps':    200,
     'limit_low':         True,
 }