[AZ-595] Batch 75: sitl_observer FDR-replay + scenario probe cleanup

Implement all 11 `sitl_observer` public surfaces as an offline
FDR-replay strategy (reads JSON fixtures under `${E2E_SITL_REPLAY_DIR}`
instead of live pymavlink/yamspy). Replace 12 per-scenario
`_harness_helpers_implemented` probes with one shared session-scoped
`sitl_replay_ready` fixture in `e2e/tests/conftest.py`.

Net: -636 LoC of duplicated scenario gating, +17 LoC shared fixture,
+38 new unit tests (596 total, up from 558). Includes K=3 cumulative
review for batches 73-75 (PASS).

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

e2e/tests/positive/test_ft_p_02_derkachi_drift.py

@@ -23,16 +23,16 @@ What this file does NOT own:
   (still a stub; AZ-408 was about the synthetic-injection injectors,
   not the video replayer); the scenario is marked
   ``@pytest.mark.deferred_ac(reason=...)`` until that helper lands.
-* The FDR-archive iteration → ``runner.helpers.fdr_reader`` (owned by
-  AZ-441); same skip gate.
+* The FDR-archive iteration → ``runner.helpers.fdr_reader`` (AZ-594,
+  landed in batch 74) — the scenario still depends on a prepared SITL
+  replay fixture (AZ-595) that produces the per-run FDR archive.
 * The MAVLink ``GLOBAL_POSITION_INT`` GT replay → handled by the
-  ``imu_replay`` helper which currently raises NotImplementedError
-  (owned by AZ-407 in spec, but the helper file was not touched by
-  the AZ-407 batch).
+  ``imu_replay`` helper (AZ-594, landed in batch 74).
 
-When all three upstream helpers land, this file's runtime path activates
-automatically — the skip is keyed off the ``NotImplementedError`` from
-the helper imports, not off a hard-coded marker.
+When ``E2E_SITL_REPLAY_DIR`` is set and points at a prepared SITL
+replay fixture, this file's runtime path activates automatically; until
+then the scenario skips via the shared `sitl_replay_ready` fixture
+(AZ-595).
 """
 
 from __future__ import annotations
@@ -44,53 +44,6 @@ import pytest
 from runner.helpers import anchor_pair_detector as apd
 
 
-@pytest.fixture(scope="module")
-def _harness_helpers_implemented() -> bool:
-    """True iff every upstream helper FT-P-02 needs has a real impl.
-
-    Used to gate the full-replay scenarios. Helper-level NotImplementedError
-    is the signal — we don't hard-code a "deferred until task X" marker
-    because then a developer who lands the helper would have to also
-    remember to flip the marker. The auto-detect pattern is also what
-    other downstream scenarios will reuse.
-    """
-    from runner.helpers import fdr_reader, frame_source_replay, imu_replay
-    from runner.helpers.frame_source_replay import FrameSourceReplayer
-    try:
-        # The cheapest sentinel for each helper:
-        # - FrameSourceReplayer.replay_video raises NotImplementedError
-        # - fdr_reader.iter_records raises NotImplementedError
-        # - ImuReplayer.replay raises NotImplementedError
-        # We check by inspecting __doc__ / source rather than calling, so
-        # the gate stays cheap.
-        replayer = FrameSourceReplayer(sink=_NullSink())  # type: ignore[arg-type]
-        try:
-            replayer.replay_video(Path("/tmp/non-existent.mp4"))
-        except NotImplementedError:
-            return False
-        try:
-            list(fdr_reader.iter_records(Path("/tmp/non-existent")))
-        except NotImplementedError:
-            return False
-        try:
-            imu_replay.ImuReplayer(emitter=_NullImuEmitter()).replay(Path("/tmp/non-existent.csv"))  # type: ignore[arg-type]
-        except NotImplementedError:
-            return False
-        return True
-    except Exception:
-        return False
-
-
-class _NullSink:
-    def write_frame(self, jpeg_bytes: bytes, timestamp_ms: int) -> None:
-        return None
-
-
-class _NullImuEmitter:
-    def emit(self, sample: object) -> None:
-        return None
-
-
 @pytest.mark.traces_to("AC-1.3,AC-1,AC-2,AC-3,AC-4,AC-5")
 def test_ft_p_02_derkachi_drift(
     fc_adapter: str,
@@ -98,7 +51,7 @@ def test_ft_p_02_derkachi_drift(
     evidence_dir,  # type: ignore[no-untyped-def]
     run_id: str,
     nfr_recorder,  # type: ignore[no-untyped-def]
-    _harness_helpers_implemented: bool,
+    sitl_replay_ready: bool,
 ) -> None:
     """Full FT-P-02 scenario (AC-1.3). See module docstring.
 
@@ -110,11 +63,11 @@ def test_ft_p_02_derkachi_drift(
     AC-4: bin medians monotonic with age — covered by check_monotonic().
     AC-5: parametrized across (fc_adapter, vio_strategy).
     """
-    if not _harness_helpers_implemented:
+    if not sitl_replay_ready:
         pytest.skip(
-            "FT-P-02 full replay requires runner.helpers.{frame_source_replay,"
-            "fdr_reader,imu_replay} — currently AZ-441 / AZ-407 leftovers. "
-            "Pure-logic ACs covered by e2e/_unit_tests/helpers/test_anchor_pair_detector.py."
+            "FT-P-02 full replay requires `E2E_SITL_REPLAY_DIR` to point at a "
+            "prepared SITL replay fixture (AZ-595). Pure-logic ACs covered by "
+            "e2e/_unit_tests/helpers/test_anchor_pair_detector.py."
         )
 
     # Once the helpers land, the body below activates. We keep it