feat: Add driver_class override pattern for platform-specific BLE drivers

Enable subclassing BLEInterface with custom platform-specific drivers by
  introducing a class-level driver_class attribute that can be overridden.

  Changes:
  - Import LinuxBluetoothDriver optionally with HAS_LINUX_DRIVER flag
  - Add driver_class class attribute (defaults to LinuxBluetoothDriver)
  - Check driver_class is not None before instantiation
  - Use self.driver_class() instead of hardcoded LinuxBluetoothDriver()
  - Log which driver is being used at initialization

  This pattern enables platform-specific implementations like:
    class AndroidBLEInterface(BLEInterface):
        driver_class = AndroidBLEDriver

  Without this pattern, subclasses would need to override __init__ entirely
  to use a different driver, duplicating all initialization logic.

  Implementation details:
  - LinuxBluetoothDriver import wrapped in try/except with fallback to None
  - Raises ImportError if driver_class is None and no override provided
  - Maintains backward compatibility (LinuxBluetoothDriver used by default)
  - All production features preserved (logging redirect, blacklist, rate
    limiting, service UUID filtering, connection management)

  Use case:
  This pattern is used by the Columba Android app to integrate the Android
  BLE stack via Chaquopy, overriding driver_class with AndroidBLEDriver
  that bridges to Kotlin BLE APIs.

  Testing:
  - Default behavior unchanged (uses LinuxBluetoothDriver)
  - Subclass override tested in columba/python/android_ble_interface.py
  - No functional changes to existing BLE interface behavior

src/RNS/Interfaces/BLEInterface.py

@@ -104,11 +104,17 @@ try:
 except ImportError:
     from RNS.Interfaces.bluetooth_driver import BLEDriverInterface, BLEDevice
 
-# Import platform-specific driver
+# Import platform-specific driver (optional - can be overridden by subclasses)
 try:
     from linux_bluetooth_driver import LinuxBluetoothDriver
+    HAS_LINUX_DRIVER = True
 except ImportError:
-    from RNS.Interfaces.linux_bluetooth_driver import LinuxBluetoothDriver
+    try:
+        from RNS.Interfaces.linux_bluetooth_driver import LinuxBluetoothDriver
+        HAS_LINUX_DRIVER = True
+    except ImportError:
+        HAS_LINUX_DRIVER = False
+        LinuxBluetoothDriver = None
 
 HAS_DRIVER = True
 
@@ -258,6 +264,9 @@ class BLEInterface(Interface):
     FRAG_TYPE_END = 0x03
     FRAG_HEADER_SIZE = 5  # bytes: type(1) + sequence(2) + total(2)
 
+    # Platform-specific driver class (override in subclasses for different platforms)
+    driver_class = LinuxBluetoothDriver
+
     def __init__(self, owner, configuration):
         """
         Initialize BLE interface.
@@ -358,8 +367,14 @@ class BLEInterface(Interface):
 
         # Discovery state with prioritization
 
-        # Initialize BLE driver
-        self.driver = LinuxBluetoothDriver(
+        # Initialize BLE driver (uses class attribute, can be overridden by subclasses)
+        if self.driver_class is None:
+            raise ImportError(
+                "No BLE driver available. LinuxBluetoothDriver not found and no "
+                "driver_class override provided by subclass."
+            )
+
+        self.driver = self.driver_class(
             discovery_interval=self.discovery_interval,
             connection_timeout=self.connection_timeout,
             min_rssi=self.min_rssi,
@@ -367,6 +382,7 @@ class BLEInterface(Interface):
             max_peers=self.max_peers,
             adapter_index=0  # TODO: Make configurable
         )
+        RNS.log(f"{self} Using driver: {type(self.driver).__name__}", RNS.LOG_DEBUG)
 
         # Set driver callbacks
         self.driver.on_device_discovered = self._device_discovered_callback