Show roadside units in the station list; log ESP32 drop counters

Bench test on 2026-08-25 against live RSU and CiT One traffic found that 611 RSU
CAMs decoded correctly and none of them were ever displayed. Excluding RSUs from
UseCaseDetectionEngine - correct, since a permanently stationary station at a
fixed point trips the stopped-vehicle use case for as long as it is in range -
also removed them from the map and station list, because remotePositions is the
engine's own map.

RSUs are now tracked in a separate rsuStations flow and merged with the engine's
road users for display only. Expiry is clock-driven for the same reason as
hazards and signals: an RSU going out of range simply stops transmitting, and no
further emission would arrive to recompute the list. Cleared on link-down
alongside engine.reset(), so a stale RSU cannot outlive an unplug.

The kinematics line is suppressed for them. An RSU's CAM uses
rsuContainerHighFrequency, which carries no kinematics at all, so the zeroes in
the model are placeholders - printing "0.0 km/h - heading 0" would assert a
stationary vehicle pointing due north.

Also logs the ESP32's STATUS heartbeat counters whenever one changes. They
previously reached only the CAM Pinger card, so a bench run captured through
logcat had no record of whether the firmware dropped anything. Logged on change
rather than per beat: the interesting event is a drop appearing, and a
once-per-second line would bury it.

Test report in 05-obu-bench-test-2026-08-25.md.

app/src/main/java/com/hawhamburg/micr0bu/MainActivity.kt

@@ -324,7 +324,7 @@ class MainActivity : AppCompatActivity() {
                             // hiltViewModel() — that would create a separate instance scoped to
                             // this NavBackStackEntry, whose onCleared() (fired the moment you
                             // navigate away) would disconnect the shared UsbSerialTransport out
-                            // from under every other screen still using it.
+                            // from under every other screen still using.
                             ConnectionSetupScreen(viewModel = mqttViewModel)
                         }
                         composable(Screen.Map.route) {