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.
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.
