feat(testing): per-frame JSONL trace in E2EHarness

Opt-in trace_path parameter dumps one JSON record per processed frame
with the fields diagnostics need:

  frame_idx, timestamp_ns, vo_success, alignment_success,
  tracking_state, confidence,
  eskf_initialized, eskf_position_enu (or None), eskf_pos_sigma_m,
  estimate_lat/lon, gt_lat/lon/alt

No perf cost when trace_path is None. File is rotated per run — safe to
point at /tmp/foo.jsonl for ad-hoc debugging.

First real run on EuRoC MH_01 (100 frames) immediately exposes the
concrete divergence: vo_success=0/100 (VO never engages on EuRoC
grayscale imagery with current SP+LG adapter), eskf_initialized=0/100,
alignment_success=77/100 (satellite-fallback path fires). Diagnosis
that was hidden behind a single "ATE=10.9 km" number is now machine-
readable per frame.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/gps_denied/testing/harness.py

@@ -13,7 +13,9 @@ What the harness does NOT do:
 
 from __future__ import annotations
 
+import json
 from dataclasses import dataclass, field
+from pathlib import Path
 from typing import Optional
 from unittest.mock import AsyncMock, MagicMock
 
@@ -29,7 +31,11 @@ from gps_denied.core.processor import FlightProcessor
 from gps_denied.core.recovery import FailureRecoveryCoordinator
 from gps_denied.core.vo import SequentialVisualOdometry
 from gps_denied.schemas.graph import FactorGraphConfig
-from gps_denied.testing.datasets.base import DatasetAdapter, PlatformClass
+from gps_denied.testing.datasets.base import (
+    DatasetAdapter,
+    DatasetPose,
+    PlatformClass,
+)
 
 EARTH_R = 6_378_137.0
 
@@ -59,10 +65,12 @@ class E2EHarness:
         adapter: DatasetAdapter,
         flight_id: str = "e2e-flight",
         max_frames: Optional[int] = None,
+        trace_path: Optional[Path] = None,
     ) -> None:
         self._adapter = adapter
         self._flight_id = flight_id
         self._max_frames = max_frames
+        self._trace_path: Optional[Path] = Path(trace_path) if trace_path else None
         self._estimates: list[tuple[int, Optional[tuple[float, float, float]]]] = []
 
     async def run(self) -> HarnessResult:
@@ -74,15 +82,35 @@ class E2EHarness:
             frames = frames[: self._max_frames]
             gt_poses = gt_poses[: self._max_frames]
 
-        for frame in frames:
-            image = self._load_or_synth_image(frame.image_path)
-            result = await processor.process_frame(
-                self._flight_id, frame.frame_idx, image
-            )
-            est = None
-            if result.gps is not None:
-                est = (result.gps.lat, result.gps.lon, 0.0)  # alt not returned here
-            self._estimates.append((frame.frame_idx, est))
+        # Align GT by index so trace records can pair each frame with the
+        # corresponding pose without an expensive timestamp search.
+        gt_by_idx: dict[int, DatasetPose] = {}
+        for i, pose in enumerate(gt_poses):
+            gt_by_idx[i] = pose
+
+        trace_fh = None
+        if self._trace_path is not None:
+            self._trace_path.parent.mkdir(parents=True, exist_ok=True)
+            trace_fh = self._trace_path.open("w")
+
+        try:
+            for frame in frames:
+                image = self._load_or_synth_image(frame.image_path)
+                result = await processor.process_frame(
+                    self._flight_id, frame.frame_idx, image
+                )
+                est = None
+                if result.gps is not None:
+                    est = (result.gps.lat, result.gps.lon, 0.0)
+                self._estimates.append((frame.frame_idx, est))
+
+                if trace_fh is not None:
+                    gt = gt_by_idx.get(frame.frame_idx)
+                    record = self._trace_record(processor, frame, result, gt)
+                    trace_fh.write(json.dumps(record) + "\n")
+        finally:
+            if trace_fh is not None:
+                trace_fh.close()
 
         gt_enu = self._poses_to_enu(gt_poses)
         est_enu = self._estimates_to_enu(gt_poses[0] if gt_poses else None)
@@ -96,6 +124,51 @@ class E2EHarness:
             platform_class=self._adapter.capabilities.platform_class,
         )
 
+    def _trace_record(
+        self,
+        processor: FlightProcessor,
+        frame,
+        result,
+        gt: Optional[DatasetPose],
+    ) -> dict:
+        """Build one JSONL record describing the product's state after a frame.
+
+        Captures VO success, ESKF state (position + trace(cov) as a scalar
+        sigma), and the estimate/GT pair. Enough to diagnose *where* the
+        pipeline diverges without dumping raw images or covariance matrices.
+        """
+        eskf = processor._eskf.get(self._flight_id)  # noqa: SLF001 — test harness
+        eskf_initialized = bool(eskf and eskf.initialized)
+        eskf_position = None
+        eskf_sigma = None
+        if eskf_initialized:
+            pos = eskf.position
+            eskf_position = [float(pos[0]), float(pos[1]), float(pos[2])]
+            cov = eskf.covariance
+            if cov is not None:
+                # Scalar horizontal+vertical position uncertainty summary.
+                eskf_sigma = float(np.sqrt(np.trace(cov[0:3, 0:3]) / 3.0))
+
+        est_lat = result.gps.lat if result.gps is not None else None
+        est_lon = result.gps.lon if result.gps is not None else None
+
+        return {
+            "frame_idx": frame.frame_idx,
+            "timestamp_ns": frame.timestamp_ns,
+            "vo_success": bool(result.vo_success),
+            "alignment_success": bool(result.alignment_success),
+            "tracking_state": result.tracking_state.value,
+            "confidence": float(result.confidence),
+            "eskf_initialized": eskf_initialized,
+            "eskf_position_enu": eskf_position,
+            "eskf_pos_sigma_m": eskf_sigma,
+            "estimate_lat": est_lat,
+            "estimate_lon": est_lon,
+            "gt_lat": gt.lat if gt else None,
+            "gt_lon": gt.lon if gt else None,
+            "gt_alt": gt.alt if gt else None,
+        }
+
     def _build_processor(self) -> FlightProcessor:
         repo = MagicMock()
         streamer = MagicMock()

tests/e2e/test_harness_smoke.py

@@ -64,3 +64,61 @@ async def test_harness_max_frames_larger_than_dataset_is_harmless():
     harness = E2EHarness(adapter, max_frames=100)
     result = await harness.run()
     assert result.num_frames_submitted == 4
+
+
+@pytest.mark.asyncio
+async def test_harness_trace_path_none_does_not_emit(tmp_path):
+    # No trace_path → no file created, no perf cost from serialisation.
+    adapter = SyntheticAdapter(num_frames=3, fps=5.0)
+    harness = E2EHarness(adapter)
+    await harness.run()
+    # Sanity: tmp_path stays empty (harness never touches it).
+    assert list(tmp_path.iterdir()) == []
+
+
+@pytest.mark.asyncio
+async def test_harness_trace_path_writes_jsonl_with_expected_fields(tmp_path):
+    import json
+
+    trace = tmp_path / "run.jsonl"
+    adapter = SyntheticAdapter(num_frames=4, fps=5.0)
+    harness = E2EHarness(adapter, trace_path=trace)
+    await harness.run()
+
+    assert trace.is_file()
+    lines = trace.read_text().splitlines()
+    # One JSON record per submitted frame
+    assert len(lines) == 4
+
+    first = json.loads(lines[0])
+    # Contract: every record carries these keys so downstream tooling can
+    # depend on them across datasets.
+    expected_keys = {
+        "frame_idx",
+        "timestamp_ns",
+        "vo_success",
+        "alignment_success",
+        "tracking_state",
+        "confidence",
+        "eskf_initialized",
+        "eskf_position_enu",     # list[3] or None when not initialized
+        "eskf_pos_sigma_m",      # float or None
+        "estimate_lat",          # float or None
+        "estimate_lon",
+        "gt_lat",
+        "gt_lon",
+        "gt_alt",
+    }
+    missing = expected_keys - set(first.keys())
+    assert not missing, f"trace record missing keys: {missing}"
+
+
+@pytest.mark.asyncio
+async def test_harness_trace_truncates_with_max_frames(tmp_path):
+    # When max_frames caps iteration, trace file has exactly that many lines.
+    trace = tmp_path / "run.jsonl"
+    adapter = SyntheticAdapter(num_frames=10, fps=5.0)
+    harness = E2EHarness(adapter, max_frames=3, trace_path=trace)
+    await harness.run()
+    lines = trace.read_text().splitlines()
+    assert len(lines) == 3