fix: force OBD reconnect after ~20s of stale null speed (BUG-24)

obd_speed() now counts consecutive null-speed polls. If the ELM327 returns
null for 10 polls (~20s) while still reporting CAR_CONNECTED, it force-closes
the connection so the reconnect loop re-handshakes instead of silently
flatlining until the next ignition cycle.

modules/app_obd.py

@@ -11,6 +11,9 @@ from modules import app_events, app_gps
 
 RECONNECT_INTERVAL = 10
 SENSOR_POLL_INTERVAL = 5
+# Speed poll is every 2s; if the adapter returns null for this many consecutive
+# polls (~20s) the ELM327 is usually stalled — force a reconnect.
+STALE_SPEED_RECONNECT_THRESHOLD = 10
 
 status = None
 speed = 0
@@ -357,14 +360,25 @@ def get_fresh_dtc_codes(max_age=SENSOR_POLL_INTERVAL * 3):
 def obd_speed():
     global speed, speed_updated_at
     with obd_speed_lock:
+        null_streak = 0
         while not stop_speed.is_set() and _is_car_connected():
             response = _query(obd.commands.SPEED)
             if response is None:
                 break
             if response.is_null():
+                null_streak += 1
                 print("No speed data received. #" + str(get_speedtime()))
                 print(app_gps.get_gps())
+                # A stalled ELM327 keeps the connection "alive" but returns
+                # null for speed forever. Force a reconnect after ~20s of null
+                # reads instead of flatlining until the next ignition cycle.
+                if null_streak >= STALE_SPEED_RECONNECT_THRESHOLD:
+                    with _connection_lock:
+                        candidate = connection
+                    _mark_connection_lost(candidate, "speed", "stale null speed")
+                    break
             else:
+                null_streak = 0
                 speed = round(response.value.to("kph").magnitude, 1)
                 speed_updated_at = time.monotonic()
                 print("Speed:", speed, "kph")