Replace time.sleep() with threading.Event.wait() in all poll
loops so worker threads exit immediately on shutdown instead of
blocking for up to 500ms per iteration. Fixes the on_unmount
crash (NoMatches from querying removed DOM nodes) by signaling
the event directly rather than iterating child widgets.
Three shutdown paths covered: q key (on_unmount), Ctrl+C
(try/finally in main), and Textual internal shutdown.
Poll threads (position at 2 Hz, signal when monitoring) run
while-loops with time.sleep() that never exit on app quit.
Add on_unmount() to PositionScreen, SignalScreen, and BirdcageApp
to clear polling flags and disconnect the device, so worker threads
exit within the sleep interval instead of hitting the 300s timeout.
F1 Position (compass rose, motor control, sparklines),
F2 Signal (RSSI gauge with sub-char precision, DVB/ADC sparklines, LNA toggle),
F3 Scan (AZ/EL grid sweep with heatmap and CSV export),
F4 System (NVS table, A3981 diagnostics, motor dynamics),
F5 Console (raw serial terminal with prompt detection and safety gates).
Includes SerialBridge (thread-safe protocol wrapper), DemoDevice
(synthetic simulation for --demo mode), dark RF theme with rounded
borders and teal accents, and send_raw() on CarryoutG2Protocol.
