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/klipper-macros.cfg

@@ -20,6 +20,7 @@
 # Printing
 [include print/pause_resume.cfg]
 [include print/filament.cfg]
+[include print/nozzle_guard.cfg]        # bd_pressure nozzle collision guard (needs extras/bdp_guard.py)
 [include print/homing.cfg]
 [include print/idle_timeout.cfg]
 [include print/print_start_end.cfg]