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>

CLAUDE.md

@@ -70,6 +70,11 @@ detection, `filament_error_sensitivity`), iHeater chamber heater.
   not `action_raise_error` / `RESPOND TYPE=error`, so they also show on the
   display. `action_raise_error` aborts while rendering, so an `M117` in the
   same macro would never run. No double quotes in `MSG`/`DISPLAY`.
+- Nozzle guard (`print/nozzle_guard.cfg`, `extras/bdp_guard.py`): wrap only
+  moves where nothing may touch the nozzle in `_GUARD_ON PHASE=x` /
+  `_GUARD_OFF`. Never leave it on across contact homing, extrusion, purges,
+  the Blobifier park or AFC cuts and loads: in `mode: stop` that is an
+  instant shutdown.
 - After a refactor run `python tools/effective_config.py HEAD` and check that
   only the intended sections changed.
 - Render-test macros with Jinja2 using Klipper's delimiters