bdp_guard: one lock for all I2C transfers of the bd_pressure

The guard's timer read the sensor while G28 ran; bdpressure.py writes the
probe mode at every homing move, and Klipper allows only one I2C request
at a time per device: KeyError ('i2c_response', 1), Klipper shut down
during HOME. The guard now wraps the sensor's i2c in a reactor mutex at
connect; bdpressure and bdpressure_check (now looks up sensor.i2c at
every use) go through it too.

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

README.md

@@ -54,7 +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. |
+| `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. Puts a lock on the sensor's I2C connection that `bdpressure.py` and `bdpressure_check.py` also go through (Klipper allows one I2C request at a time per device). Needs the load-check firmware. |
 
 ### Scripts (`scripts/`)