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/nozzle_guard.cfg

@@ -0,0 +1,33 @@
+# Nozzle collision guard: bd_pressure watches the nozzle force during moves
+# that must not touch anything (needs extras/bdp_guard.py and the bd_pressure
+# load-check firmware). See extras/bdp_guard.py.
+#
+# mode: log only measures and reports each phase (console and klippy.log,
+# "bdp_guard:"). Run a few print starts in log mode, set threshold well
+# above the peaks the phases show, then switch to mode: stop.
+
+[bdp_guard]
+sensor: bd_pa
+mode: log
+threshold: 30
+samples: 3
+interval: 0.02
+
+# M400 first: BDP_GUARD_ARM/DISARM act at once, not in the move queue, so
+# without it the guard would watch moves that are still queued (contact
+# homing, extrusion) or stop watching before the armed moves ran.
+[gcode_macro _GUARD_ON]
+description: Arm the nozzle guard for PHASE=<name> [MODE=log|stop] after the queued moves (nothing without [bdp_guard])
+gcode:
+    {% if 'bdp_guard' in printer %}
+      M400
+      BDP_GUARD_ARM PHASE={params.PHASE|default('armed')} {('MODE=' ~ params.MODE) if 'MODE' in params else ''}
+    {% endif %}
+
+[gcode_macro _GUARD_OFF]
+description: Disarm the nozzle guard after the queued moves (nothing without [bdp_guard])
+gcode:
+    {% if 'bdp_guard' in printer %}
+      M400
+      BDP_GUARD_DISARM
+    {% endif %}