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>

README.md

@@ -24,7 +24,8 @@ everything below, grouped by topic. Comment out what you do not use there.
 | `print_start_end.cfg` | `PRINT_WARMUP`, `PRINT_START`, `PRINT_END` | Beacon, AFC, Blobifier, `HEAT_SOAK`, iHeater, bd_pressure PA macros (bdwidth is no longer enabled while printing) |
 | `pause_resume.cfg` | `PAUSE`, `RESUME`, `CANCEL_PRINT`, `M600` (RESUME also handles a running `HEAT_SOAK`, and after a PAUSE purges 40 mm with `BLOBIFIER_CHECKED` first: a failed check keeps the print paused, `RESUME PURGE=0` skips the purge; CANCEL_PRINT runs the same cleanup as `PRINT_END`) | `heating/heat_soak.cfg`, `leds/`, `blobifier/`, `afc/purge_check.cfg` |
 | `filament.cfg` | `LOAD_FILAMENT`, `UNLOAD_FILAMENT` | — |
-| `homing.cfg` | `HOME` | — |
+| `homing.cfg` | `HOME`: X/Y, then Z always with Beacon contact (also when already homed); `_Z_CHECK`: stops the print when Klipper's Z and the Beacon reading differ (run in `PRINT_START` after the soak) | Beacon |
+| `nozzle_guard.cfg` | `[bdp_guard]`, `_GUARD_ON PHASE=<name>`, `_GUARD_OFF`: bd_pressure watches the nozzle force during print-start moves where nothing should touch it (QGL, Z homing, mesh); `mode: log` only reports the peak per phase, `mode: stop` shuts Klipper down on a touch | `extras/bdp_guard.py` + bd_pressure load-check firmware |
 | `idle_timeout.cfg` | `[idle_timeout]`: heaters, chamber heater and motors off | iHeater macros |
 | **heating/** | | |
 | `heat_soak.cfg` | `HEAT_SOAK`, `STOP_HEAT_SOAK`, `CANCEL_HEAT_SOAK`, `HEAT_SOAK_RESUME`, `RESUME` | — |
@@ -53,6 +54,7 @@ everything below, grouped by topic. Comment out what you do not use there.
 | `xy_drift.py` | `AFC_XY_SAFETY_CHECK`: measures lost XY steps from the stepper step counts at the endstop triggers (CoreXY). The toolhead position after homing cannot show lost steps. |
 | `bdwidth_status.py` | exposes bdwidth's motion counter and width to macros (`present: False` with neutral values when no bdwidth is configured, so the macros also work without it), and keeps a USB dropout of bdwidth (port: usb) from shutting down Klipper: the read is retried every 10 s and the port reopened |
 | `bdpressure_check.py` | bd_pressure load check (`BDP_CHECK_START/STOP/QUERY`). Needs the load-check firmware from the bd_pressure `load-check-extension` branch; on stock firmware it reports "not available" and does nothing. Also reports how many readings the last `PA_CALIBRATE` got (`pa_points`, `pa_distinct`), used to reject runs with a silent or stuck sensor. |
+| `bdp_guard.py` | nozzle collision guard: while armed (`BDP_GUARD_ARM/DISARM/QUERY`) reads the bd_pressure force every 20 ms over I2C and, in stop mode, shuts Klipper down when it stays above the threshold. Reports reads, peak and errors per phase in the console and klippy.log. Needs the load-check firmware. |
 
 ### Scripts (`scripts/`)