This fixes two issues that caused BLE connections to hang on macOS
when not using the --debug flag:

1. Race condition in BLEClient event loop initialization
   - The event loop thread was started but asyncio operations were
     submitted before the loop was actually running
   - Added threading.Event synchronization to ensure the event loop
     is running before any operations are submitted
   - The ready signal is sent from within the loop via call_soon()
     to guarantee the loop is truly active

2. CoreBluetooth callback delivery on macOS
   - On macOS, CoreBluetooth requires occasional I/O operations for
     callbacks to be properly delivered to the main thread
   - Without --debug, no I/O was happening, causing callbacks to
     never be processed and operations to hang indefinitely
   - Added sys.stdout.flush() call before waiting for async results
     to trigger the necessary I/O

The --debug flag masked these issues because:
- Debug logging introduces timing delays that let the event loop start
- Logger I/O triggers the necessary callback delivery mechanism

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Aleksei Sviridkin <f@lex.la>