[AZ-598] Batch 78: sitl_observer.wait_for_outbound + FT-P-01 fixture builder

Phase 1: extend sitl_observer with cursor-based `wait_for_outbound`
returning `OutboundMessage` from `outbound_messages_<fc_kind>_<host>.json`
fixtures. Three outcomes: message, TimeoutError (null entries), or
RuntimeError (missing/malformed). Fix FT-P-01 + FT-P-05 scenarios to
use `fc_kind=` kwarg.

Phase 2: FT-P-01 vertical-slice fixture builder under
`e2e/fixtures/sitl_replay_builder/`. Reuses the production
`gps-denied-replay` CLI + `ReplayInputAdapter`: encode 60 stills as
1 fps MP4 + synthetic stationary tlog (pymavlink); run replay;
project FDR outbound estimates into the schema. Avoids the
13+ cp of SUT-side frame-ingestion that a live-SITL-capture path
would have required. Live execution remains a manual operator step.

+35 unit tests (664 total, up from 637). K=3 cumulative review for
b76-b78 documents the offline-replay arc convergence.

Co-authored-by: Cursor <cursoragent@cursor.com>

e2e/_unit_tests/helpers/test_sitl_observer.py

@@ -211,6 +211,204 @@ def test_get_observer_missing_gps_state_raises(replay_dir: Path):
         obs.read_gps_state()
 
 
+# wait_for_outbound (AZ-598)
+
+
+def _write_observer_fixture(replay_dir: Path, fc_kind: str, host: str) -> None:
+    """Write the minimal `observer_<kind>_<host>.json` so `get_observer` succeeds."""
+    _write_json(
+        replay_dir / f"observer_{fc_kind}_{host}.json",
+        {
+            "gps_state": {
+                "primary_source": "MAV",
+                "last_position_lat_deg": 0.0,
+                "last_position_lon_deg": 0.0,
+                "last_position_alt_m": 0.0,
+                "fix_quality": 3,
+                "horizontal_accuracy_m": 1.0,
+                "last_update_age_ms": 0,
+            },
+            "parameters": {},
+        },
+    )
+
+
+def test_wait_for_outbound_advances_cursor_in_order(replay_dir: Path):
+    # Arrange
+    _write_observer_fixture(replay_dir, "ardupilot", "sitl-host")
+    _write_json(
+        replay_dir / "outbound_messages_ardupilot_sitl-host.json",
+        {
+            "messages": [
+                {"image_id": "AD000001.jpg", "lat_deg": 48.275292, "lon_deg": 37.385220},
+                {"image_id": "AD000002.jpg", "lat_deg": 48.275001, "lon_deg": 37.382922},
+            ]
+        },
+    )
+    obs = so.get_observer("ardupilot", "sitl-host")
+
+    # Act
+    first = obs.wait_for_outbound(timeout_s=5.0)
+    second = obs.wait_for_outbound(timeout_s=5.0)
+
+    # Assert
+    assert first.lat_deg == 48.275292 and first.lon_deg == 37.385220
+    assert first.image_id == "AD000001.jpg"
+    assert second.lat_deg == 48.275001 and second.lon_deg == 37.382922
+    assert second.image_id == "AD000002.jpg"
+
+
+def test_wait_for_outbound_null_entry_raises_timeout(replay_dir: Path):
+    # Arrange
+    _write_observer_fixture(replay_dir, "ardupilot", "sitl-host")
+    _write_json(
+        replay_dir / "outbound_messages_ardupilot_sitl-host.json",
+        {"messages": [None]},
+    )
+    obs = so.get_observer("ardupilot", "sitl-host")
+
+    # Assert
+    with pytest.raises(TimeoutError, match="captured as timeout in fixture"):
+        obs.wait_for_outbound(timeout_s=5.0)
+
+
+def test_wait_for_outbound_advances_cursor_past_timeout(replay_dir: Path):
+    # Arrange — a real timeout in the middle of the sequence does not stall
+    # the cursor; the next call advances normally.
+    _write_observer_fixture(replay_dir, "ardupilot", "sitl-host")
+    _write_json(
+        replay_dir / "outbound_messages_ardupilot_sitl-host.json",
+        {
+            "messages": [
+                {"lat_deg": 1.0, "lon_deg": 2.0},
+                None,
+                {"lat_deg": 3.0, "lon_deg": 4.0},
+            ]
+        },
+    )
+    obs = so.get_observer("ardupilot", "sitl-host")
+
+    # Act / Assert
+    assert obs.wait_for_outbound().lat_deg == 1.0
+    with pytest.raises(TimeoutError):
+        obs.wait_for_outbound()
+    third = obs.wait_for_outbound()
+    assert third.lat_deg == 3.0 and third.lon_deg == 4.0
+
+
+def test_wait_for_outbound_exhausted_raises_runtime(replay_dir: Path):
+    # Arrange
+    _write_observer_fixture(replay_dir, "ardupilot", "sitl-host")
+    _write_json(
+        replay_dir / "outbound_messages_ardupilot_sitl-host.json",
+        {"messages": [{"lat_deg": 1.0, "lon_deg": 2.0}]},
+    )
+    obs = so.get_observer("ardupilot", "sitl-host")
+    obs.wait_for_outbound()  # drain the only entry
+
+    # Assert
+    with pytest.raises(RuntimeError, match="outbound messages fixture exhausted"):
+        obs.wait_for_outbound()
+
+
+def test_wait_for_outbound_missing_fixture_raises_runtime(replay_dir: Path):
+    # Arrange — observer fixture present, outbound fixture missing.
+    _write_observer_fixture(replay_dir, "ardupilot", "sitl-host")
+    obs = so.get_observer("ardupilot", "sitl-host")
+
+    # Assert
+    with pytest.raises(RuntimeError, match="outbound_messages_ardupilot_sitl-host.json"):
+        obs.wait_for_outbound()
+
+
+def test_wait_for_outbound_missing_env_raises_runtime(unset_replay_dir):
+    # Arrange — observer dataclass constructed manually so we don't depend on env var
+    # for the observer-fixture load. Verifies the outbound load itself respects the env.
+    obs = so._FdrReplayObserver(fc_kind="ardupilot", host="sitl-host", _payload={})
+
+    # Assert
+    with pytest.raises(RuntimeError, match="env var not set"):
+        obs.wait_for_outbound()
+
+
+def test_wait_for_outbound_messages_not_list_raises_runtime(replay_dir: Path):
+    # Arrange
+    _write_observer_fixture(replay_dir, "ardupilot", "sitl-host")
+    _write_json(
+        replay_dir / "outbound_messages_ardupilot_sitl-host.json",
+        {"messages": {"oops": "should be list"}},
+    )
+    obs = so.get_observer("ardupilot", "sitl-host")
+
+    # Assert
+    with pytest.raises(RuntimeError, match="`messages` must be a JSON list"):
+        obs.wait_for_outbound()
+
+
+def test_wait_for_outbound_entry_wrong_type_raises_runtime(replay_dir: Path):
+    # Arrange
+    _write_observer_fixture(replay_dir, "ardupilot", "sitl-host")
+    _write_json(
+        replay_dir / "outbound_messages_ardupilot_sitl-host.json",
+        {"messages": ["not-an-object"]},
+    )
+    obs = so.get_observer("ardupilot", "sitl-host")
+
+    # Assert
+    with pytest.raises(RuntimeError, match=r"messages\[0\] must be a JSON object or null"):
+        obs.wait_for_outbound()
+
+
+def test_wait_for_outbound_entry_missing_coords_raises_runtime(replay_dir: Path):
+    # Arrange
+    _write_observer_fixture(replay_dir, "ardupilot", "sitl-host")
+    _write_json(
+        replay_dir / "outbound_messages_ardupilot_sitl-host.json",
+        {"messages": [{"image_id": "AD000001.jpg"}]},
+    )
+    obs = so.get_observer("ardupilot", "sitl-host")
+
+    # Assert
+    with pytest.raises(RuntimeError, match="missing required `lat_deg`/`lon_deg`"):
+        obs.wait_for_outbound()
+
+
+def test_wait_for_outbound_image_id_optional(replay_dir: Path):
+    # Arrange — entries without `image_id` are valid; consumer only needs coords.
+    _write_observer_fixture(replay_dir, "ardupilot", "sitl-host")
+    _write_json(
+        replay_dir / "outbound_messages_ardupilot_sitl-host.json",
+        {"messages": [{"lat_deg": 10.0, "lon_deg": 20.0}]},
+    )
+    obs = so.get_observer("ardupilot", "sitl-host")
+
+    # Act
+    msg = obs.wait_for_outbound()
+
+    # Assert
+    assert msg.lat_deg == 10.0 and msg.lon_deg == 20.0
+    assert msg.image_id is None
+
+
+def test_wait_for_outbound_separate_observers_have_independent_cursors(replay_dir: Path):
+    # Arrange — two observers built from the same fixture file must NOT share cursor.
+    _write_observer_fixture(replay_dir, "ardupilot", "sitl-host")
+    _write_json(
+        replay_dir / "outbound_messages_ardupilot_sitl-host.json",
+        {"messages": [{"lat_deg": 1.0, "lon_deg": 2.0}, {"lat_deg": 3.0, "lon_deg": 4.0}]},
+    )
+
+    # Act
+    obs_a = so.get_observer("ardupilot", "sitl-host")
+    obs_b = so.get_observer("ardupilot", "sitl-host")
+    a_first = obs_a.wait_for_outbound()
+    b_first = obs_b.wait_for_outbound()
+
+    # Assert
+    assert a_first.lat_deg == 1.0
+    assert b_first.lat_deg == 1.0
+
+
 # prepare_sitl_*