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>

extras/bdpressure_check.py

@@ -35,7 +35,6 @@ class BDPressureCheck:
     def __init__(self, config):
         self.printer = config.get_printer()
         self.sensor_name = 'bdpressure ' + config.get('sensor', 'bd_pa')
-        self.i2c = None
         self.sensor = None
         self.status = {'available': False, 'running': False, 'peak': 0,
                        'above_ms': 0, 'longest_ms': 0, 'delta': 0}
@@ -50,9 +49,13 @@ class BDPressureCheck:
                                desc="Read the bd_pressure load check values")
 
     def _handle_connect(self):
-        sensor = self.printer.lookup_object(self.sensor_name)
-        self.sensor = sensor
-        self.i2c = getattr(sensor, 'i2c', None)
+        self.sensor = self.printer.lookup_object(self.sensor_name)
+
+    @property
+    def i2c(self):
+        # Looked up at every use: bdp_guard replaces the sensor's i2c with a
+        # locked wrapper at connect, possibly after this module's connect.
+        return getattr(self.sensor, 'i2c', None)
 
     def _read(self):
         if self.i2c is None: