Print start: contact homing, nozzle guard, Z check, purge before chamber heat

- HOME always homes Z with Beacon contact and recalibrates, also when the
  printer is already homed (home_method_when_homed = proximity used the
  model of an earlier print without touching the bed). PRINT_START homes
  Z again after QGL, before the brush takes the nozzle below Z0.
- _Z_CHECK: after the soak, compare Klipper's Z with a BEACON_QUERY
  reading at Z3 over the bed centre and stop the print above 1 mm.
  Warns and continues when BEACON_QUERY leaves no new
  printer.beacon.last_sample.
- Nozzle guard (extras/bdp_guard.py, print/nozzle_guard.cfg): reads the
  bd_pressure force over I2C every 20 ms while armed and, in stop mode,
  shuts Klipper down on a touch. Armed for HOME X/Y, QGL, Z homing, brush,
  mesh and the Z check; starts in log mode to collect peaks per phase.
- Chamber prints: load and purge the first tool before the chamber heats
  (_LOAD_FOR_CHAMBER replaces _UNLOAD_FOR_CHAMBER). A cut unload leaves a
  piece of PLA in the extruder that softened in the hot chamber and jammed
  the ASA load.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

macros/print/homing.cfg

@@ -1,8 +1,78 @@
 # HOME: home all axes and move to the bed centre
+#
+# Z always with Beacon contact (the nozzle touches the bed), also when the
+# printer is already homed. Without METHOD=CONTACT, [beacon]
+# home_method_when_homed = proximity homes a homed printer on the
+# eddy-current model of an earlier print (other bed temperature, maybe
+# another plate) without touching the bed and without recalibrating.
+# Contact refuses a nozzle above 180C (contact_max_hotend_temperature).
 
 [gcode_macro HOME]
 gcode:
     G90
     G4 P2000
-    G28 X0 Y0 Z0
+    _GUARD_ON PHASE=home_xy
+    G28 X Y
+    # Contact homing presses the nozzle on the bed: guard off
+    _GUARD_OFF
+    G28 Z METHOD=CONTACT CALIBRATE=1
+    _GUARD_ON PHASE=home_lift
     G1 X175 Y175 Z10 F1200
+    _GUARD_OFF
+
+# _Z_CHECK: compare where Klipper thinks the nozzle is with what the Beacon
+# measures (proximity), at X/Y (default the bed centre) and Z (default 3 mm,
+# inside the model range 0.2-5 mm). Catches a Z that changed since the last
+# homing (lost Z steps, gantry moved, much thermal drift) before the nozzle
+# goes low; a difference above MAX stops the print. A wrong Beacon model
+# fools both readings the same way, so this does not replace contact homing.
+#
+# BEACON_QUERY stores its reading in printer.beacon.last_sample. When it
+# gives no new reading there, the check only warns, so a Beacon version
+# without that field does not stop every print.
+[gcode_macro _Z_CHECK]
+description: Compare Klipper's Z with the Beacon reading at X Y Z (default 175 175 3), stop the print when they differ by more than MAX (default 0.5 mm)
+gcode:
+    {% set x = params.X|default(175)|float %}
+    {% set y = params.Y|default(175)|float %}
+    {% set z = params.Z|default(3)|float %}
+    {% set max = params.MAX|default(0.5)|float %}
+    {% if 'xyz' not in printer.toolhead.homed_axes %}
+      _ERROR MSG="_Z_CHECK: printer not homed" DISPLAY="Z check: not homed"
+    {% elif 'beacon' not in printer %}
+      _WARN MSG="_Z_CHECK: no [beacon], check skipped"
+    {% else %}
+      {% set last = printer.beacon.last_sample %}
+      {% set prev = last.time if last and last.time is defined else -1 %}
+      SAVE_GCODE_STATE NAME=_z_check
+      G90
+      _GUARD_ON PHASE=z_check
+      {% if printer.toolhead.position.z < z %}
+        G1 Z{z} F600
+      {% endif %}
+      G1 X{x} Y{y} F12000
+      G1 Z{z} F600
+      _GUARD_OFF
+      G4 P300
+      BEACON_QUERY
+      _Z_CHECK_EVAL MAX={max} PREV={prev}
+      RESTORE_GCODE_STATE NAME=_z_check
+    {% endif %}
+
+# Separate macro: rendered after BEACON_QUERY has run. toolhead.position is
+# the real nozzle height (gcode offset and mesh included).
+[gcode_macro _Z_CHECK_EVAL]
+description: Helper for _Z_CHECK: compare the BEACON_QUERY reading with the toolhead Z
+gcode:
+    {% set s = printer.beacon.last_sample %}
+    {% set z = printer.toolhead.position.z %}
+    {% set max = params.MAX|default(0.5)|float %}
+    {% if not s or s.time is not defined or s.dist is not defined or s.time == params.PREV|default(-1)|float %}
+      _WARN MSG="_Z_CHECK: BEACON_QUERY left no new reading in printer.beacon.last_sample, Z check skipped" DISPLAY="Z check skipped"
+    {% elif s.dist is none %}
+      _ERROR MSG="_Z_CHECK: Klipper Z={'%.2f' % z} but the Beacon reads nothing (more than 5 mm away or no model), print stopped" DISPLAY="Z check failed, print stopped"
+    {% elif (s.dist - z)|abs > max %}
+      _ERROR MSG="_Z_CHECK: Klipper Z={'%.3f' % z}, Beacon measures {'%.3f' % s.dist} mm (difference {'%.3f' % (s.dist - z)}, limit {max}), print stopped" DISPLAY="Z check failed, print stopped"
+    {% else %}
+      RESPOND MSG="Z check: Klipper Z={'%.3f' % z}, Beacon {'%.3f' % s.dist} mm, difference {'%.3f' % (s.dist - z)} (limit {max})"
+    {% endif %}