Keep every element of a list-shaped event

17 events are MAP_ITEM or LIST_ITEM: they arrive once per element, all
carrying the same argument names, so a flat store kept only whichever landed
last. SensorsStatesListChanged reports six sensors that way, and the
simulator's deliberate magnetometer fault was invisible through state() as a
result, which is exactly the fact preflight_check needs to see.

Entries are now keyed Command[element]_arg. Parrot marks these events but
never names the key; it is the first argument by convention.

tests/test_list_events.py

@@ -0,0 +1,52 @@
+"""List-shaped events must not overwrite each other.
+
+MAP_ITEM and LIST_ITEM events arrive once per element with identical argument
+names. A flat store keeps only the last, which silently hides exactly the kind
+of fact a preflight check looks for.
+"""
+
+import asyncio
+
+import pytest
+
+from mcbebop.arsdk.session import DroneSession
+from mcbebop.protocol import xml_index
+from mcbebop.sim import FakeBebop
+
+SENSORS = "common.CommonState.SensorsStatesListChanged"
+
+
+def test_list_events_are_indexed():
+    listed = [c for c in xml_index.all_commands() if c.list_key]
+    assert len(listed) == 17, "both XML files together define 17 MAP_ITEM/LIST_ITEM events"
+    assert xml_index.get(SENSORS).list_key == "sensorName"
+    # A scalar event must not be treated as a list.
+    assert xml_index.get("common.CommonState.BatteryStateChanged").list_key is None
+
+
+def test_every_sensor_survives_and_a_fault_is_visible():
+    async def run():
+        with FakeBebop() as sim:
+            session = DroneSession(ip=sim.host, discovery_port=sim.discovery_port)
+            await session.connect()
+            await session.request_full_state()
+            await asyncio.sleep(2.0)
+            values = session.values()
+            await session.disconnect()
+            return values
+
+    values = asyncio.run(run())
+    sensors = {
+        key.split("[", 1)[1].split("]", 1)[0]: value
+        for key, value in values.items()
+        if key.startswith("SensorsStatesListChanged[") and key.endswith("_sensorState")
+    }
+    assert len(sensors) == 6, f"expected all six self-tests, got {sorted(sensors)}"
+    faults = sorted(name for name, ok in sensors.items() if not ok)
+    assert faults == ["magnetometer"], "the simulator's deliberate fault must be reachable"
+
+
+@pytest.mark.parametrize("name", [SENSORS, "common.CommonState.MassStorageInfoStateListChanged"])
+def test_list_key_is_the_first_argument(name):
+    spec = xml_index.get(name)
+    assert spec.list_key == spec.args[0].name